Light‐curve inversions with truncated least‐squares principal components: Tests and application to HD 291095 = V1355 Orionis

We present a new inversion code that reconstructs the stellar surface spot configuration from the light curve of a rotating star. Our code employs a method that uses the truncated least‐squares estimation of the inverse problem's objects principal components. We use spot filling factors as the unknown objects. Various test cases that represent a rapidly‐rotating K subgiant are used for the forward problem. Tests are then performed to recover the artificial input map and include data errors and input‐parameter errors. We demonstrate the robustness of the solution to false input parameters like photospheric temperature, spot temperature, gravity, inclination, unspotted brightness and different spot distributions and we also demonstrate the insensitivity of the solution to spot latitude. Tests with spots peppered over the entire stellar surface or with phase gaps do not produce fake active longitudes. The code is then applied to ten years of V and I ‐band light curve data of the spotted sub‐giant HD291095. A total of 22 light curves is presented. We find that for most of the time its spots were grouped around two active longitudes separated on average by 180°. Switches of the dominant active region between these two longitudes likely occurred about every 3.15±0.23 years while the amplitude modulation of the brightness occurred with a possible period of 3.0±0.15 years. For the first time, we found evidence that the times of the activity flips coincide with times of minimum light as well as minimum photometric amplitude, i.e. maximum spottedness. From a comparison with simultaneous Doppler images we conclude that the activity flips likely take place near the rotational pole of the star. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spot activity on HD 89546 (FG UMa) from long‐term photometry

We present the analysis of 20 years of time‐series BV photometry of the SB1 RS CVn binary HD 89546. The system's yearly mean V brightness, the B – V color index, the photometric period, and the light curve amplitude all show clear cyclic variability with an ≈9‐year time scale. We also find some evidence for brightness variability on a time scale longer than the 20‐year time span of our observations, perhaps indicating a longer cycle analogous to the solar Gleissberg cycle. We estimate the unspotted V magnitude of HD 89546 to be 7.^m154, which is ≈0.^m2 brighter than the observed maximum brightness. Spot modelling of the system shows that spot temperature variations affect the observed B – V color as well as the V brightness. Two active longitudes are observed, centered around 180° and 360° longitude on the G9 III primary, each covering a longitude range of 120°. Furthermore, two inactive longitude zones are seen spanning only 60° between the two active longitudes. The longitudinal distribution of the spots exhibits no strong cyclic variability but does show rapid jumps of 120° that look like the flip‐flop phenomenon. We estimate the differential rotation coefficient of the star as k = 0.086 by considering the range of observed photometric period variations and assumed latitudinal spot variations over 45° (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photometric and spectroscopic variability of the pre‐main sequence star V1184 Tauri (CB 34V)

We present recent results from optical photometric and spectroscopic observations of the pre‐main sequence star V1184 Tau (CB 34V). The star is associated with the Bok globule CB 34 and was considered as a FUOR candidate in previous studies. Our photometric data obtained from October 2000 to April 2003 show that the stellar brightness varies with an amplitude of about 0.m 5 (I ), but from August 2003 the photometric behavior of the star has changed dramatically. Three deep brightness minima (ΔI ∼ 4^m.2) were observed during the past two years. The analysis of available photometric data suggests that V1184 Tau shows two types of variability produced (1) by rotation of large cool spotted surface and (2) by occultation from circumstellar clouds of dust or from features of a circumstellar disk. The behavior of the V – I index indicates that the star becomes redder towards minimum light, but from a certain turning point (V ∼ 18^m.2) it gets bluer and is fading further. Five medium dispersion optical spectra of V1184 Tau were obtained in the period 2001–2004. Signi.cant changes in the profile and strength of the emission lines in the spectrum of V1184 Tau were found. During minimum light the equivalent width of the Hα emission line increases from 4 Å to 9 Å. The [O I] lines (λλ 6003, 6363 Å) are also seen in emission while the sodium doublet keeps its absorption strength and equivalent width. The possibility to reconstruct the historical light curve of V1184 Tau using photographical plate archives is brie.y discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photometric monitoring of the young star Par 1724 in Orion

We report new photometric observations of the ∼200 000 year old naked weak‐line run‐away T Tauri star Par 1724, located north of the Trapezium cluster in Orion. We observed in the broad band filters B, V, R, and I using the 90 cm Dutch telescope on La Silla, the 80 cm Wendelstein telescope, and a 25 cm telescope of the University Observatory Jena in Großschwabhausen near Jena. The photometric data in V and R are consistent with a ∼5.7 day rotation period due to spots, as observed before between 1960ies and 2000. Also, for the first time, we present evidence for a long‐term 9 or 17.5 year cycle in photometric data (V band) of such a young star, a cycle similar to that to of the Sun and other active stars (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A photometric study of the contact binary AZ Virginis

New light curves and photometric solutions of the contact binary AZ Vir are presented in this paper. The light curves appear to exhibit a typical O'Connell effect, with Maximum I being 0.021 mag (V) and 0.023 mag (B) brighter than Maximum II, respectively. From the observations, six times of minimum light were determined and from the present times of minimum light and those collected from the references, the light elements of the system were improved. The light curves were analyzed by means of the Wilson‐Devinney program. The results suggest that AZ Vir is a W‐subtype contact binary with a mass ratio of q = 0.623(2). The asymmetry of the light curves is explained by star spot models. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

CK Bootis: a W UMa system with a small mass ratio

We present an analysis of BV R light curves of an eclipsing binary CK Bootis, a system with a very small mass ratio. The light curves appear to exhibit a typical O'Connell effect. The light curves are analyzed by means of the latest version of the WD program. The asymmetry of the light curves is explained by a cool star spot model. The simultaneous BV R synthetic light curve analysis gives a tiny mass ratio of 0.12, an extremely large fill‐out factor of 0.65, and a very small difference between the component temperatures of 90 K. The absolute parameters of the system were also derived by combining the photometric solutions with the radial velocity data. The mass of the secondary is very low (0.15 M_⊙) and it continues losing mass. Thirty seven new times of minimum are reported. It is found that the orbital period of the system has a quasi periodic variation, superimposed on a period increase. The long‐term period increase rate is deduced to be dP/dt = 3.54x10^–7 d yr^–1, which can be interpreted as being due to mass transfer from the less massive star to the more massive component. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New observations,photometric study and preliminary elements of the W UMa system DF CVn

CCD (V) light curve of the EW‐type eclipsing variable DF CVn was obtained during seven nights in April–May, 2004. With our data we were able to determine 4 new times of minimum light. The light curve appears to exhibit a typical O'Connell effect, with Maximum I brighter than Maximum II by 0.013 mag. in V. TwoWilson‐Devinney (WD) code working sessions, using the V light curve, were done with and without spots. The analysis of the results shows that the best fit was obtained with the spotted solution and indicates contact geometry. The photometric mass ratio of the system is found to be q = 0.347 and its inclination i = 72°, the primary minimum shows a transit. The star may be classified as an A‐type W Uma system. Assuming a reasonable value for the mass of the primary component an estimate of the absolute elements of DF CVn has been made, with the assumption that the primary has a mass corresponding to its spectral type according to Straizys and Kuriliene (1981). (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Is the semi‐regular variable RU Vulpeculae undergoing a helium‐shell flash?

The semi‐regular variable star RU Vulpeculae (RU Vul) is being observed visually since 1935. Its pulsation period and amplitude are declining since ∼1954. A leading hypothesis to explain the period decrease in asymptotic giant branch (AGB) stars such as RU Vul is an ongoing flash of the He‐burning shell, also called a thermal pulse (TP), inside the star. In this paper, we present a CCD photometric light curve of RU Vul, derive its fundamental parameters, and test if the TP hypothesis can describe the observed period decline. We use CCD photometry to determine the present‐day pulsation period and amplitude in three photometric bands, and high‐resolution optical spectroscopy to derive the fundamental parameters. The period evolution of RU Vul is compared to predictions by evolutionary models of the AGB phase. We find that RU Vul is a metal‐poor star with a metallicity [M/H] = –1.59 ± 0.05 and an effective surface temperature of T_eff = 3634 ± 20 K. The low metallicity of RU Vul and its kinematics indicate that it is an old, low‐mass member of the thick disc or the halo population. The present day pulsation period determined from our photometry is ∼108 d, the semiamplitude in the V ‐band is 0.39 ± 0.03 mag. The observed period decline is found to be well matched by an evolutionary AGB model with stellar parameters comparable to those of RU Vul. We conclude that the TP hypothesis is in good agreement with the observed period evolution of RU Vul. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photometric study of the active binary star V1430 Aquilae

New BVR light curves and a photometric analysis of the eclipsing binary star V1430 Aql are presented. The light curves were obtained at the Çanakkale Onsekiz Mart University Observatory in 2004. The light curves are generally those of detached eclipsing binaries, but there are large asymmetries between maxima. New BVR light curves were analysed with an ILOT procedure. Light curve asymmetries of the system were explained in terms of large dark starspots on the primary component. The primary star shows a long‐lived and quasi‐poloidal spot distribution with active longitudes in opposite hemispheres. Absolute parameters of the system were derived.We also discuss the evolution of the system: the components are likely to be pre‐main sequence stars, but a post‐main sequence stage cannot be ruled out. More observations are needed to decide this point. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Discovery of cyclic spot activity on the G8 giant HD 208472

We present and analyze 17 consecutive years of UBVRI time‐series photometry of the spotted giant component of the RS CVn binary HD 208472. Our aim is to determine the morphology and the evolution of its starspots by using periodsearch techniques and two‐spot light‐curve modelling. Spots on HD208472 always occur on hemispheres facing the observer during orbital quadrature and flip their location to the opposite hemisphere every approximately six years. The times when the spots change their preferential hemisphere correspond to times when the light curve amplitudes are the smallest and when abrupt changes of the photometric periods are observed. During these times the star is also close to a relative maximum brightness, suggesting a vanishing overall spottedness at each end of the previous cycle and the start of a new one. We find evidence for a 6.28±0.06‐yr brightness cycle, which we interpret to be a stellar analog of the solar 11‐year sunspot cycle. We also present clear evidence for a brightening trend, approximated with a 21.5±0.5‐yr period, possibly due to a stellar analog of the solar Gleissberg cycle. From the two‐spot modelling we also determine an upper limit for the differential‐rotation coefficient of α = ΔP/P of 0.004±0.010, which would be fifty times weaker than on the Sun (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Time series photometric spot modelling – V. Phase coherence of spots on UZ Librae

We present spot models for nine years of continuous VI_c photometry of UZ Lib from 1993–2001. The relatively stable double‐humped light curve shape suggests extreme phase coherence. From the spot‐modelling analysis, we found that the major spots or spot groups are always located on the hemisphere facing the secondary star and exactly in the opposite hemisphere anti‐facing the secondary. Several single‐humped light curves and our suggested binary scenario rule out a pure ellipsoidal variability as the cause of the double‐humped light curve shape. We try to explain this preferred spot pattern with a magnetic‐field structure that connects the two components, as suggested earlier for RS CVn stars in general. A possible 4.8 years spot cycle is found from the long‐term brightness variations but needs confirmation. We rediscuss the basic astrophysical data of UZ Lib. The Hipparcos parallax is likely wrong, a possible reason could be that UZ Lib is e.g., a triple system.     

New photometry and astrometry of the isolated neutron star RX J0720.4–3125 using recent VLT/FORS observations

Since the first optical detection of RX J0720.4–3125 various observations have been performed to determine astrometric and photometric data. We present the first detection of the isolated neutron star in the V Bessel filter to study the spectral energy distribution and derive a new astrometric position. At ESO Paranal we obtained very deep images with FORS 1 (three hours exposure time) of RX J0720.4–3125 in the V Bessel filter in January 2008. We derive the visual magnitude by standard star aperture photometry. Using sophisticated resampling software we correct the images for field distortions. Then we derive an updated position and proper motion value by comparing its position with FORS 1 observations of December 2000. We calculate a visual magnitude of V = 26.81 ± 0.09 mag, which is seven times in excess of what is expected from X‐ray data, but consistent with the extant U, B, and R data. Over about a seven year epoch difference we measured a proper motion of μ = 105.1 ± 7.4 mas yr^–1 towards θ = 296.951° ± 0.0063° (NW), consistent with previous data (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New classification and basic stellar parameters of SU Equulei

The variable star SU Equulei was classified as a close binary with an eclipse light curve previously categorized as WUMa type. The aim of this paper is a review of this old classification on the basis of new observations and a new determination of variable star ephemeris as well as the determination of SU Equ spectral type and distance. New photometric observations in different colours allow a precise determination of the period of variability and yield more accurate light curves allowing a re‐classification of the type of variability. We find the best period of variability to be half the old value. The shape of the light curve is inconsistent with an eclipse curve but consistent with an RR Lyrae type c classification. From the B, V, and R colours we deduce a new spectral classification. SU Equulei is an RRc Lyrae type variable of spectral class A8 at a distance of ≈12.4 kpc instead of a late‐type eclipsing binary (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Long‐term photometry of the ultra‐fast rotator LO Peg

LO Peg is a young main‐sequence star of spectral type K3. With its equatorial rotation velocity of 65 km s^–1 it is amongst the ultra‐fast rotators. Its high equatorial rotation velocity and rapidly changing surface activity features make it an important object in terms of both stellar activity and the evolution of stellar rotation and angular momentum. Since its discovery as a variable star, it has mostly been subject to spectral surface mapping studies such as Doppler Imaging, while there have been very few photometric studies on it. This paper aims to present the first long‐term photometric observations and its results covering the years between 2003 and 2009. The UBVR Johnson wide band photometric data showed that the surface activity structures of LO Peg vary in timescales changing between days and months, and parallel to this, the mean, maximum and minimum brightness and amplitudes change dramatically between years and sometimes even within the same observation season. Long‐term changes in system brightness and colours, both characteristic features of active stars, were also seen in this ultra‐fast young star. The active longitudes, which has a life time of ∼1.3 years and an activity cycle period of ∼4.8 years for LO Peg were estimated (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

CCD‐Δa and BV R photometry of NGC 7296

The first CCD photometric investigation of the open cluster NGC 7296 up to now was performed within the narrow band Δa photometric system, which enables us to detect peculiar objects. A deeper investigation of that cluster followed, using the standard BV R ‐Bessel filter set. The age and E (B – V ) was determined independently to log t = 8.0 ± 0.1 and 0.15 ± 0.02, respectively by using Δa and broadband photometry. In total five Be/Ae objects and two metal‐weak stars showing significant negative Δa ‐values as well as one classical chemically peculiar star could be identified within that intermediate age open cluster. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Four‐colour photometry of EY Dra: A study of an ultra‐fast rotating active dM1‐2e star

We present more than 1000‐day long photometry of EY Draconis in BV (RI)_C passbands. The changes in the light curve are caused by the spottedness of the rotating surface. Modelling of the spotted surface shows that there are two large active regions present on the star on the opposite hemispheres. The evolution of the surface patterns suggests a flip‐flop phenomenon. Using Fourier analysis, we detect a rotation period of P_rot = 0.45875 d, and an activity cycle with P ≈ 350 d, similar to the 11‐year long cycle of the Sun. This cycle with its year‐long period is the shortest one ever detected on active stars. Two bright flares are also detected and analysed (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

HD 1: The number‐one star in the sky

We present the first ever study of the bright star HD 1. The star was chosen arbitrarily just because of its outstanding Henry Draper number. Surprisingly, almost nothing is known about this bright 7.^m4 star. Our observations were performed as part of the commissioning of the robotic telescope facility STELLA and its fiber‐fed high‐resolution optical echelle spectrograph SES in the years 2007–2010. We found long‐term radial velocity variations with a full amplitude of 9 km s^–1 with an average velocity of –29.8 km s^–1 and suggest the star to be a hitherto unknown single‐lined spectroscopic binary. A preliminary orbit with a period of 6.2 years (2279±69 days) and an eccentricity of 0.50±0.01 is given. Its rms uncertainty is just 73 m s^–1. HD 1 appears to be a G9‐K0 giant of luminosity class IIIa with T_eff = 4850±100 K, logg = 2.0±0.2, L ≈ 155 L_⊙, a mass of 3.0±0.3 M_⊙, a radius of 17.7 R_⊙, and an age of ≈350 Myr. A relative abundance analysis led to a metallicity of [Fe/H] = –0.12 ± 0.09. The α ‐element silicon may indicate an overabundance of +0.13 though. The low strengths of some s‐process lines and a lower limit for the ¹²C/¹³C isotope ratio of ≥16 indicate that HD 1 is on the first ascend of the RGB. The absorption spectral lines appear rotationally broadened with a v sin i of 5.5±1.2 km s^–1 but no chromospheric activity is evident. We also present photometric monitoring BV (RI)_C data taken in parallel with STELLA. The star is likely a small‐amplitude (<10 mmag) photometric variable although no periodicity was found (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Results of the photometry of the spotted dM1‐2e star EY Draconis

We have observed EY Draconis with the 60‐cm telescope of Konkoly Observatory in Budapest for 64 nights. In the first observing season the star produced a stable light curve for more than 60 stellar rotations, however, the light curves observed in the next season and the spot modelling show clear evidence of the evolution of the spotted stellar surface. The changes of the maximum brightness level suggests the existence of a longer period of about 300 days, which seems to be confirmed by the ROTSE archival data. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The chromospherically active binary star EI Eridani: II. Long‐term Doppler imaging

Data from 11 years of continuous spectroscopic observations of the active RS CVn‐type binary star EI Eridani – gained at NSO/McMath‐Pierce, KPNO/Coudé Feed and during the MUSICOS 98 campaign – were used to obtain 34 Doppler maps in three spectroscopic lines for 32 epochs, 28 of which are independent of each other. Various parameters are extracted from our Doppler maps: average temperature, fractional spottedness, and longitudinal and latitudinal spot‐occurrence functions. We find that none of these parameters show a distinct variation nor a correlation with the proposed activity cycle as seen from photometric long‐term observations. This suggests that the photometric brightness cycle may not necessarily be due to just a cool spot cycle. The general morphology of the spot pattern remains persistent over the whole period of 11 years. A large cap‐like polar spot was recovered from all our images. A high degree of variable activity was noticed near latitudes of ≈60–70° where the appendages of the polar spot emerged and dissolved (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Seven years photometry of the W UMa‐type eclipsing binary V2612 Oph

We present the first long‐term Johnson UBVR observations and comprehensive photometric analysis of the W UMa‐type eclipsing binary V2612 Oph. Observations in the time interval between 2003 and 2009 enabled us to reveal the seasonal and long‐term variations of the light curve. Hence, we found that the mean brightness level of the light curve shows a variation with a period of 6.7 years. Maximum and minimum brightness levels of the light curve exhibit a variation from year to year which we attribute to a solar‐like activity. The O – C variation of eclipse timings of the system shows a decreasing parabolic trend and reveals a period decrease at a rate of P = 6.27×10^‐7 day yr^‐1 with an additional low‐amplitude sinusoidal variation that has a similar period as the long‐term brightness variations. Our light curve analysis shows that the system is a W‐subtype W UMa eclipsing binary. We calculated masses and radii of the primary and secondary components as M₁ = 1.28 M_⊙, M₂ = 0.37 M_⊙ and R₁ = 1.31 R_⊙, R₂ = 0.75 R_⊙, respectively. The derived absolute photometric parameters allow us to calculate a distance of 140 pc, which confirms that the system is a foreground star in the sky field of the Galactic open cluster NGC 6633. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)