Period variation and spot model for the W UMa type binary TY UMa

We present the full VRI light curves and the times of minima of TY UMa to provide a complete photometric solution and a long-term trend of period variation. The light curves show a high degree of asymmetry (the O'Connell effect). The maxima at 0.25 phase (Max I) are 0.021, 0.015, and 0.020 mag fainter than those at 0.75 phase (Max II) in V , R , and I , respectively. The period of TY UMa has varied in a sinusoidal way, superimposed on the long-term upward parabolic variation. The secularly increasing rate of the period is deduced as 1.83 s per century  ( P˙ / P =5.788×10^-10 d d^-1)  . The period of sinusoidal variation is about 57.4 yr. The spot model has been applied to fit the asymmetric light curves of TY UMa, to explain light variations. By changing only the spot parameters, the model light curves can fit the observed light curves for three epochs. This indicates that the variation of the spot location and size is the main reason for changing the shape of light curves, including two different maxima and the interchanging depths of occultation and transit minima.     

BVR observations and period variation of the near-contact binary ZZ Aurigae

BVR light curves of ZZ Aurigae were obtained with the 60-cm Cassegrain reflector at the Sobaek Observatory, Korea, between 2000 February and 2001 February. All collected times of minimum light, including our observations, were used for the period study. The period variation could be of quasi-sinusoidal form superposed on an upward parabola. A continuous period increase of  d P /d t =+2.3 × 10⁻⁸ d yr⁻¹  was determined for ZZ Aur. The period of quasi-sinusoidal variation is about ∼26–31 yr. Photometric solutions were found using the Wilson–Devinney method. The Roche configuration of ZZ Aur is that of an Algol-type semidetached system where the primary star nearly fills its Roche lobe and the secondary star fills its lobe. The spot model was used to explain the asymmetry in the light curve known as the O'Connell effect.     

Discovery of irradiation-induced variations in the light curve of the classical nova V2275 Cyg (N Cyg 2001 No. 2)

We present charge-coupled device (CCD) photometry, light curve and time-series analysis of the classical nova V2275 Cyg (N Cyg 2001 No. 2). The source was observed for 14 nights in total in 2002 and 2003 using an R filter with the 1.5-m Russian–Turkish joint telescope (RTT150) at the TUBITAK National Observatory in Antalya, Turkey, as part of a large programme on the CCD photometry of cataclysmic variables. We report the detection of two distinct periodicities in the light curve of the nova: (a)   P ₁= 0.314 49(15) d [7.6 h]  , and (b)   P ₂= 0.017 079(17) d [24.6 min]  . The first period is evident in both 2002 and 2003 whereas the second period is only detected in the 2003 data set. We interpret the first period as the orbital period of the system and attribute the orbital variations to aspect changes of the secondary irradiated by the hot white dwarf (WD). We suggest that the nova was a supersoft X-ray source in 2002 and, perhaps, in 2003. The second period could be a quasi-periodic oscillation originating from the oscillation of the ionization front (due to a hot WD) below the inner Lagrange point or a beat frequency in the system as a result of the magnetic nature of the WD if steady accretion has already been re-established.     

V440 Per: the longest-period overtone Cepheid

V440 Per is a Population I Cepheid with a period of 7.57 d and low-amplitude, almost sinusoidal light and radial velocity curves. With no reliable data on the first harmonic, its pulsation mode identification remained controversial. We obtained a radial velocity curve of V440 Per with our new high-precision and high-throughput Poznań Spectroscopic Telescope. Our data reach an accuracy of 130 m s⁻¹ per individual measurement and yield a secure detection of the first harmonic with an amplitude of   A ₂= 140 ± 15 m s⁻¹  . The velocity Fourier phase φ₂₁ of V440 Per is inconsistent at the 7.25σ level with those of fundamental-mode Cepheids, implying that the star must be an overtone Cepheid, as originally proposed by Kienzle et al. Thus, V440 Per becomes the longest-period Cepheid with securely established overtone pulsations. We show that a convective non-linear pulsation hydrocode can reproduce the Fourier parameters of V440 Per very well. The requirement to match the observed properties of V440 Per constrains the free parameters of the dynamical convection model used in the pulsation calculations, in particular the radiative loss parameter.     

A mystery solved: the mass ratio of the dwarf nova EM Cygni

We have discovered that the spectrum of the well-known dwarf nova EM Cyg is contaminated by light from a K25V star (in addition to the K-type mass donor star). The K25V star contributes approximately 16 per cent of the light from the system and if not taken into account has a considerable effect upon radial velocity measurements of the mass donor star. We obtain a new radial velocity amplitude for the mass donor star of K ₂=202±3 km s¹, compared with the value of K ₂=135±3 km s¹ obtained in Stover, Robinson & Nather's classic study of EM Cyg. The revised value of the amplitude, combined with a measurement of rotational broadening of the mass donor, v  sin  i =140±6 km s¹, leads to a new mass ratio of q M ₂ M ₁=0.88±0.05. This solves a long-standing problem with EM Cyg, because Stover et al.'s measurements indicated a mass ratio q >1, a value that should have led to dynamically unstable mass transfer for the secondary mass deduced by Stover et al. The revised value of the mass ratio, combined with the orbital inclination i =67±2°, leads to masses of 0.99±0.12 M and 1.12±0.08 M for the mass donor and white dwarf respectively. The mass donor is evolved, because it has a later spectral type (K3) than its mass would imply.
We discuss whether the K star could be physically associated with EM Cyg or not, and present the results of the spectroscopic study.     

Photometric modelling of starspots – II. The fortran code spotpic

A fortran code which computes synthetic light and colour curves of active, spotted stars has been developed. The main feature of this code is that it can simultaneously model the V light curve and the ( V − R )_c, ( V − I )_c, ( V − K ) colour data. It also uses new effective temperature–colour and Barnes–Evans-like calibrations, temperature and gravity-dependent limb darkening coefficients and different effective surface gravities for the spotted and unspotted photosphere. The code allows for two-component spots, i.e. spots with umbral and penumbral components. Various problematic spot configurations were investigated and we conclude that, in order to be able to differentiate spots with various thermal structures (umbrae, penumbrae, faculae) or polar spots from equatorial bands, the modelling of the infrared colours, especially ( V − I )_c and ( V − K ), is needed.     

AD Cancri：A Contact Binary with Components in Poor Thermal Contact

We present the light curve and photometric solutions of the contact binary AD Cnc. The light curve appears to exhibit a typical O'Connell effect, with Maximum I brighter than Maximum II by 0.010 mag. in V. From 1987 to 2000, the light curve showed changes of shape: the depth of the primary eclipse increased by about 0.056m while that of the secondary eclipse decreased by about 0.032m, so the difference between the primary and the secondary eclipses increased by about 0.088m, while there was no obvious variation in the O'Connell effect. Using the present and past times of minimum light, the changes in the orbital period of the system are analyzed. The result reveals that the orbital period of AD Cnc has continuously increased at a rate of dp/dt = 4.4 ×10-7day yr-1. The light curve is analyzed by means of the latest version of the Wilson-Devinney code. The results show that AD Cnc is a W-subtype contact binary with a small mass ratio of 0.267 and the two components are in poor thermal contact. AD Cnc has     

Improved astrophysical parameters for the overcontact binary FG Hydrae

Photometric data on FG Hya obtained in 2002 and 2004 are presented. Three data sets show the exchange between A-type, W-type and the variable O'Connell effects. The photometric mass ratio  ( q = 0.1115 ± 0.0003)  derived from B and V light curves is almost the same as the spectroscopic mass ratio  ( q _sp= 0.112 ± 0.004)  . The new photometric solutions reveal that FG Hya is a deep overcontact binary system  ( f = 85.6 ± 1.8 per cent)  with a spotted massive component. A period investigation, based on all available photoelectric or CCD times of light minimum, shows that the O–C curve of FG Hya can be explained as a combination of a secular period decrease and a cyclic variation with a period of 36.4 yr and an amplitude of 0.0289 d. By comparing the variation of the depth of the primary minimum with the change of the cyclic period, it is discovered that both of them may vary with the same cycle length of 36.4 yr and in the same phase. The variation of the light curve, the spotted primary component and the connection between the cyclic period change and the depth of the primary minimum, all may suggest that the G0-type component displays solar-type magnetic activity with a 36.4-yr cycle length. The long-time period decrease is interpreted by mass transfer from the more massive component to the less massive one or/and angular momentum loss due to mass outflow from the outer Lagrangian point.     

Detection of non-radial pulsation and faint companion in the symbiotic star CH Cyg

We have detected asymmetry in the symbiotic star CH Cyg through the measurement of precision closure phase with the Integrated Optics Near-Infrared Camera (IONIC) beam combiner, at the infrared optical telescope array interferometer. The position of the asymmetry changes with time and is correlated with the phase of the 2.1-year period found in the radial velocity measurements for this star. We can model the time-dependent asymmetry either as the orbit of a low-mass companion around the M giant or as an asymmetric, 20 per cent change in brightness across the M giant. We do not detect a change in the size of the star during a 3-year monitoring period neither with respect to time nor with respect to wavelength. We find a spherical dust shell with an emission size of 2.2 ± 0.1 D _* full width at half-maximum around the M giant star. The star to dust flux ratio is estimated to be 11.63 ± 0.3. While the most likely explanation for the 20 per cent change in brightness is non-radial pulsation, we argue that a low-mass companion in close orbit could be the physical cause of the pulsation. The combined effect of pulsation and low-mass companion could explain the behaviour revealed by the radial velocity curves and the time-dependent asymmetry detected in the closure-phase data. If CH Cyg is a typical long secondary period variable then these variations could be explained by the effect of an orbiting low-mass companion on the primary star.     

Photometric modelling of starspots – I. A Barnes–Evans-like surface brightness–colour relation using (Ic−K)

In the first part of this work, the empirical correlation of stellar surface brightness F _V with ( I _c− K ) broad-band colour is investigated by using a sample of stars cooler than the Sun. A bilinear correlation is found to represent well the brightness of G, K and M giant stars. The change in slope occurs at ( I _c− K )∼2.1 or at about the transition from K to M spectral types. The same relationship is also investigated for dwarf stars and found to be distinctly different from that of the giants. The dwarf star correlation differs by an average of −0.4 in ( I _c− K ) or by a maximum in F _V of ∼−0.1, positioning it below that of the giants, with both trends tending towards convergence for the hotter stars in our sample. The flux distribution derived from the F _V −( I _c− K ) relationship for the giant stars, together with that derived from an F _V −( V − K ) relationship and the blackbody flux distribution, is then utilized to compute synthetic light V and colour ( V − R )_c, ( V − I )_c and ( V − K ) curves of cool spotted stars. We investigate the effects on the amplitudes of the curves by using these F _V –colour relations and by assuming the effective gravity of the spots to be lower than the gravity of the unspotted photosphere. We find that the amplitudes produced by using the F _V −( I _c− K ) relationship are larger than those produced by the other two brightness correlations, meaning smaller and/or warmer spots.     

Spot patterns and differential rotation in the eclipsing pre-cataclysmic variable binary, V471 Tau

We present surface spot maps of the K2V primary star in the pre-cataclysmic variable binary system, V471 Tau. The spot maps show the presence of large high-latitude spots located at the sub-white dwarf longitude region. By tracking the relative movement of spot groups over the course of four nights (eight rotation cycles), we measure the surface differential rotation rate of the system. Our results reveal that the star is rotating rigidly with a surface shear rate,  dΩ= 1.6 ± 6 mrad d⁻¹  . The single active star AB Dor has a similar spectral type, rotation period and activity level as the K star in V471 Tau, but displays much stronger surface shear  (46 < dΩ < 58 mrad d⁻¹)  . Our results suggest that tidal locking may inhibit differential rotation; this reduced shear, however, does not affect the overall magnetic activity levels in active K dwarfs.     

A new method for determining physical parameters of fundamental mode RR Lyrae stars from multicolour light curves

We present a new method for determining physical parameters of RRab variables exclusively from multicolour light curves. Our method is an inverse photometric Baade–Wesselink analysis which, using a non-linear least-squares algorithm, searches for the effective temperature ( T _eff) and pulsational velocity ( V _p) curves and other physical parameters that best fit the observed light curves, utilizing synthetic colours and bolometric corrections from static atmosphere models. The T _eff and V _p curves are initially derived from empirical relations then they are varied by the fitting algorithm. The method yields the variations and the absolute values of the radius, the effective temperature, the visual brightness and the luminosity of individual objects. Distance and mass are also determined. The method is tested on nine RRab stars subjected to Baade–Wesselink analyses earlier by several authors. The physical parameters derived by our method using only the light-curve data of these stars are well within their possible ranges defined by direct Baade–Wesselink and other techniques. A new empirical relation between the I _C magnitude and the pulsational velocity is also presented, which allows to construct the V _p curve of an RRab star purely from photometric observations to an accuracy of about 3.5 km s⁻¹.     

KPD 0422 + 5421: a new short-period subdwarf B/white dwarf binary

The subdwarf B (sdB) star KPD 0422 + 5421 was discovered to be a single-lined spectroscopic binary with a period of P  = 0.090 1795 ± (3 × 10⁻⁷) d (2 h 10 min). The U B light curves display an ellipsoidal modulation with amplitudes of ≈ 0.02 mag. The sdB star contributes nearly all of the observed flux. This and the absence of any reflection effect suggest that the unseen companion star is small (i.e. R _comp ≈ 0.01 R_⊙) and therefore degenerate. We modelled the U B light curves and derived i  = 78.05° ± 0.50° and a mass ratio of q  =  M _comp/ M _sdB = 0.87 ± 0.15. The sdB star fills 69 per cent of its Roche lobe. These quantities may be combined with the mass function of the companion [ f ( M ) = 0.126 ± 0.028 M_⊙] to derive M _sdB = 0.72 ± 0.26 M_⊙ and M _comp = 0.62 ±  0.18 M_⊙. We used model spectra to derive the effective temperature, surface gravity and helium abundance of the sdB star. We found T _eff = 25 000 ± 1500 K, log g  = 5.4 ± 0.1 and [He/H] = −1.0. With a period of 2 h 10 min, KPD 0422 + 5421 has one of the shortest known orbital periods of a detached binary. This system is also one of only a few known binaries that contain a subdwarf B star and a white dwarf. Thus KPD 0422 + 5421 represents a relatively unobserved, and short-lived, stage of binary star evolution.     

Hubble Space Telescope observations of SV Cam – II. First derivative light-curve modelling using phoenix and atlas model atmospheres

The variation of the specific intensity across the stellar disc is an essential input parameter in surface brightness reconstruction techniques such as Doppler imaging, where the relative intensity contributions of different surface elements are important in detecting star-spots. We use phoenix and atlas model atmospheres to model light curves derived from high precision (signal-to-noise ratio ≃ 5000) Hubble Space Telescope ( HST ) data of the eclipsing binary SV Cam (F9V+K4V), where the variation of specific intensity across the stellar disc will determine the contact points of the binary system light curve. For the first time, we use χ² comparison fits to the first derivative profiles to determine the best-fitting model atmosphere. We show the wavelength dependence of the limb darkening and that the first derivative profile is sensitive to the limb-darkening profile very close to the limb of the primary star. It is concluded that there is only a marginal difference (<1σ) between the χ² comparison fits of the two model atmospheres to the HST light curve at all wavelengths. The usefulness of the second derivative of the light curve for measuring the sharpness of the primary's limb is investigated, but we find that the data are too noisy to permit a quantitative analysis.     

A period study and light-curve synthesis for the Algol-type semidetached binary XX Cephei

We obtained CCD photometric observations of the Algol-type semidetached binary XX Cephei (XX Cep) during 15 nights from 2002 September 17 to 2003 February 2, and also on 2005 January 21. Except for those data taken on the last night of the concentrated observing season, the 3881 measurements were obtained over an interval of only 106 nights. From these data, four new times of minimum light were calculated. The  (O− C)  diagram formed from all available timings, and thus the orbital period of the system, can be partly represented as a beat effect between two cyclical variations with different periods (      yr,      yr) and amplitudes  ( K ₁=0.015 d, K ₂=0.103 d)  , respectively. Both physical and non-physical interpretations of these cycles were investigated. The long-term sinusoidal variation is too long for magnetic cycling in solar-type single and close binary stars. In addition, we have studied the effect of a possible secular period variation. By analysing the residuals from our Wilson–Devinney (WD) binary model, we found small light variations with a period of 5.99 d with amplitudes growing toward longer wavelengths. We think that these oscillations may be produced by instabilities at the systemic L ₁ point (also occupied by the point of the cool star) and that these instabilities are, in turn, caused by non-uniform and sporadic convection. There is also a short-period oscillation of about 45 min in the WD light residuals that is attributed to accretion on to the mass-gaining primary component from a feeble gas stream originating on the cool donor star.     

BL Andromedae and GW Tauri: close binary stars in a key evolutionary stage

A photoelectric light curve of BL And is presented along with the first CCD light curve of GW Tau. Both objects are short-period eclipsing binaries and were observed in 2003 or 2004. Photometric elements were computed using the latest version of the Wilson–Van Hamme code. The results reveal that BL And is a semidetached system with the primary component filling its Roche lobe and the secondary one almost filling but still detached, while GW Tau is a marginal-contact binary system with a small degree of contact ( f = 10.9 per cent) and a large temperature difference of about 3100 K. All available eclipse times, including new ones, were analysed for each system. It was found that the orbital period of BL And is decreasing at the rate of  d P /d t =−2.36 × 10⁻⁸ (±0.09) d yr⁻¹  while that of GW Tau may be decreasing or oscillating. We think period decrease is more probable. The derived configuration and secular period decrease for BL And combined with the asymmetry of the light curve indicate that this system may evolve from the present semidetached phase into a contact stage, with mass transfer from the primary component to the secondary one through the L ₁ point, or that it might just undergo the broken stage predicted by the theory of thermal relaxation oscillations. In contrast, GW Tau is a marginal-contact binary in poor thermal contact and may be at the beginning of the contact phase.     

The EC 14026 stars – VIII. PG 1336−018: a pulsating sdB star in an HW Vir-type eclipsing binary

The sdB star PG 1336−018 is found to be a very short-period eclipsing binary system, remarkably similar to the previously unique system HW Vir. In addition, and unlike HW Vir, the sdB star in the PG 1336 system shows rapid oscillations of the type found in the recently discovered sdB pulsators, or EC 14026 stars. The orbital period, 0.101 0174 d, is one of the shortest known for a detached binary. Analysis of photoelectric and CCD photometry reveals pulsation periods near 184 and 141 s, with semi-amplitudes of ∼0.01 and ∼0.005 mag respectively. Both oscillations might have variable amplitude, and it is probable that other frequencies are present with amplitudes ∼0.003 mag or less. The 184- and 141-s pulsations are in the range of periods predicted by models for hot horizontal-branch stars. Analysis of medium-dispersion spectrograms yields T _eff=33 000±1000 K and log g =5.7±0.1 for the sdB primary star, a radial velocity semi-amplitude K ₁=78±3 km s⁻¹ and a system velocity γ=6±2 km s⁻¹. Spectrograms from the IUE Final Archive give T _eff=33 000±3000 K and E ( B − V )=0.05 for log g =6.0 models. The derived angular radius leads to a distance of 710±50 pc for the system, and an absolute magnitude for the sdB star of +4.1±0.2. A preliminary analysis of U , V and R light curves indicates the orbital inclination to be near 81° and the relative radii to be r ₁=0.19 and r ₂=0.205. Assuming the mass of the sdB primary to be 0.5 M⊙ leads to a mass ratio q =0.3 for the system, and indicates that the secondary is a late-type dwarf of type ∼M5. As with HW Vir, it is necessary to invoke small limb-darkening coefficients and high albedos for the secondary star to obtain reasonable fits to the observed light curves.     

Detection of 6-cm radio-continuum emission from an EB (β-Lyrae type) variable star – HIP 68718

We report the 6-cm radio-continuum detection of a previously unknown radio star, HIP 68718, identified by the Hipparcos satellite as an EB or β-Lyrae type optical variable. Only five prior radio detections of this type are reported in the literature. The radio source (VLA J140402−002145) was detected at the 6σ level, with an associated <1 arcsec position error. The radio-optical separation is 0.2 arcsec and is well within the position errors. A ROSAT X-ray source (RXJ 1404.0−0021) is located 4.5 arcsec from the radio position. Based on the component spectral types (F5V + G5V), 1.52 d period, variations in the peak of the light curve and the X-ray and 6-cm radio luminosities, we conclude the star is a candidate RS CVn. The binary is in a near-contact phase based on the asymmetric light curve (O'Connell effect). Evidence of two faint possible companion objects was also found in the J - and K -band Two-Micron All-Sky Survey images.     

Zooming in on a sleeping giant: milliarcsecond High Sensitivity Array imaging of the black hole binary V404 Cyg in quiescence

Observations of the black hole X-ray binary V404 Cyg with the very long baseline interferometer the High Sensitivity Array (HSA) have detected the source at a frequency of 8.4 GHz, providing a source position accurate to 0.3 mas relative to the calibrator source. The observations put an upper limit of 1.3 mas on the source size (5.2 au at 4 kpc) and a lower limit of  7 × 10⁶  K on its brightness temperature during the normal quiescent state, implying that the radio emission must be non-thermal, most probably synchrotron radiation, possibly from a jet. The radio light curves show a short flare, with a rise time of ∼30 min, confirming that the source remains active in the quiescent state.     

HD 9770, a southern active-chromosphere system

The visual triple system HD 9770 (BB Scl) has been the subject of a four-year programme of UVB ( RI )_C photometry and H α échelle spectroscopy. Analysis of the data obtained over that period shows that star B, and probably also star A, of HD 9770 is a binary. The A system comprises a K1V star, which may be in a binary system with another K dwarf. The B system is an eclipsing binary of the BY Dra type in which both stars are chromospherically active. An orbital period of 0.476 525±0.000 013 d has been derived from the light curve in V . Physical parameters derived from analysis of the light curves in UBV ( RI )_C are presented.