Light curve variations in V471 Tauri

The light curves of V471 Tau obtained in four successive years from 1981 to 1984 are presented. The light variations due to the tidally distorted red dwarf have been computed and subtracted out from the light curves obtained so far. A pure shape of the wave-like distortion has therefore been derived. The period of the retrograde migration has been found to be 182.17 days. On the other hand, the mean brightness of the system seems to vary with a period of about 18 years. The amplitude of the variation is greater at longer wavelength. The periodic variation of the mean brightness resembles the solar cycle of about 11.2 years, which depends upon the number of spots or spot groups.     

Preliminary results from the spectra of V471 tauri

The findings about this unusual eclipsing variable which possesses a white-dwarf component are reported. It was found that the H and K absorption lines of Caii are strongly pressure-broadened, and exhibit two double-reversal emission components. The radial velocities derived from the central strong emission components of Caii (H and K) are in agreement with the mean radial velocity curve obtained from the metallic absorption lines of the existing elements, whereas H (4340.47 Å) does not conform with the mean radial velocity curve. In certain orbital phases the H (6562.82 Å) line occurs in emission.     

Photoelectric light and colour variations in V471 Tauri

A new investigation of the variations in the light curves and in the period of the eclipsing binary V471 Tau is presented. The collected observational data have been re-examined and, in addition to old information, it was found that (i) the decrease in the period of the system slows down and (ii) that the mean brightness of the system has been increasing and this is greater at the longer wavelength. For the last seven years the increase in the brightness is estimated to be 0.15 mag inB and 0.18 mag inV bands respectively. Therefore, we conclude that the colour of the system is about 0.03 mag redder in 1980 than in 1973.     

Light curve variations of the eclipsing binary V367 Cygni

The long-period eclipsing binary star V367 Cygni has been observed photoelectrically in two colours,B andV, in 1984, 1985, and 1986. These new light curves of the system have been discussed and compared for the light-variability with the earlier ones presented by Heiser (1962). Using some of the previously published photoelectric light curves and the present ones, several primary minima times have been derived to calculate the light elements. Any attempt to obtain a photometric solution of the binary is so complicated by the peculiar nature of the light curve caused by the presence of the circumstellar matter in the system. Despite this difficulty, however, some approaches are being carried out to solve the light curves which will be discussed shortly.     

Electron temperature and emission measure variations during solar X-ray flares

X-ray emission from seventeen X-ray flares was analyzed to obtain electron temperatures and emission measures associated with the source region in the solar corona. The source region was assumed to be isothermal with a Maxwellian electron velocity distribution.Flares which were characterized by a rapid initial X-ray flux increase were found to also have a rapid initial rise in electron temperature and emission measure. Flares which were characterized by a gradual initial X-ray energy flux increase were found to have a less rapid initial rise in electron temperature and emission measure. In all X-ray flares studied the peak temperature chronologically preceded the peak X-ray flux and the peak flux never came after the peak emission measure.Based on a dissertation submitted to The Catholic University of America, Washington, D.C.     

Extensive photometric study of V471 Tauri

The white-dwarf red-dwarf eclipsing binary V471 Tauri has been observed photoelectrically, in blue and yellow lights, from 1985 to 1989. The behaviour of the mean brightness variation has been re-examined. The mean brightness has decreased from 1982 to 1985 and thereafter it started to increase again. The amount of the variation of the light of the system has reached to 0.19 mag in both colours since 1973. On the other hand, the new timings of mid-eclipse deviate considerably from those linear and light-time effect fittings. Neither the third-body hypothesis, nor any other mechanism can explain the orbital period change of the system.     

Periodical light variations of t tauri stars as a result of disk accretion

I argue that temperatures of spots, responsible for observed periodical light variations of T Tauri stars (TTS), are not known with reliable accuracy to discriminate between chromospheric and accretion theories of TTS 's phenomenon. The hypothesis is set up that spots on classical TTS (CTTS) are due to heating of stellar surface by radiation from a collisional accretion shock, whereas spots on weak line TTS (WTTS), at least in some cases, are connected with a collisionless accretion shock rather than chromospheric activity. Possible scenarios of WTTS interaction with circumstellar matter are discussed.     

Analysis of spectra of V471 Tau and HD 115404

We analyze the chemical composition of the atmospheres of a single K-type star HD 115404 and the secondary component of the V471 Tau variable. We use the technique of modeling of synthetic spectra to analyze the high-resolution spectra of these stars, taken with the RTT 150 Russian-Turkish telescope and find the abundances of 23 and 17 elements in the atmospheres of HD 115404 and V471 Tau, respectively. We demonstrate the lack of composition anomalies in the HD 115404 and show it to be consistent with the published data, inferred from equivalent widths of spectral lines. We find the abundances of 15 elements from Na to Ba to be consistent with the metallicity of the atmosphere of V471 Tau ([Fe/H] = −0.22 ± 0.12dex), which differs significantly from the average metallicity of the Hyades cluster. We show the existence of strong carbon and oxygen overabundances (by more than 1dex) due to the enrichment of the secondary by the nucleosynthesis products during the common-envelope stage of the system. On the whole, we demonstrate that V471 Tau and the other precataclysmic variables share similar composition anomalies.     

Photometry of the contact binary system V471 Cas

UBV photoelectric observations of the W UMa-type binary V471 Cas were made from September to November 1984. Its colour indices were found to be(B-V)=0 _. ^m 771±0 _. ^m 02 and(U-B)=0 _. ^m 196±0 _. ^m 02. TheU, B, andV light curves of V471 Cas show some photometric fluctuations. We found that its orbital period is not 0.335998 days which was given by GCVS (1986), but 0.405356 days.Photometric orbital elements of V471 Cas were found using the Wilson-Devinney method. V471 Cas is a contact system, in which the overcontact factor is 0.19, its mass ratio of two components is 0.5947, and orbital minclination is in 83.29 degrees.     

Period variation and the new light curves of V471 Tauri

The light curve and period variation of the eclipsing system V471 Tau is discussed. The migration period of 191 days obtained recently by Ibanolu agrees well with the new observations. The period decreases by about one second per century, which may correspond to a mass transfer of 1×10^–7 solar mass. Furthermore, the O–C diagram shows a noticeable sine curve super-imposed on the parabolic variation. The period of the sine curve is about 3.1 years.     

Hα flares from V404 Cyg in quiescence

We present a spectrophotometric study of short-term optical variability in the quiescent black hole X-ray transient V404 Cyg. This includes two nights of high-time-resolution H α spectroscopy with which we resolve much of the time variability, and a further six nights of archival spectroscopy with lower time resolution but higher spectral resolution. We find significant variability in most of the data considered, with both the H α line and the continuum often varying in a correlated way. This includes both dramatic flares lasting a few hours in which the line flux nearly doubles and lower-level flickering. The strongest flares involve development of asymmetry in the line profile, with the red wing usually strongest independent of orbital phase. It is unclear why this is the case, but we discuss several possible explanations. We consider the energetics of the flares and compare with plausible models including chromospheric activity on the companion star, local magnetic reconnection events within the disc and varying irradiation from close to the black hole. Based on the line profile changes during the flares, we conclude that the most likely origin for the variability is variable photoionization by the central source, although local flares within the disc cannot be ruled out.     

B-V photoelectric photometry of white-dwarf eclipsing binary V471 Tauri

Photoelectric photometry of the unusual binary system, whose light curve shows peculiar light variations, has been done in the two colours. The observations obtained at the Ege University Observatory between 1973 and 1978 and at the Kottamia Observatory in November 1977 show a variable light curve. The observed variations in the light curve show a migrating wave towards a decreasing orbital phase similar to those observed in RS CVn-binaries. The migration period appears to be about 191 days. The times of minima indicate that there has been a decrease on the orbital period of the system. However, since there is not enough material on the observed times of minima, we cannot explain whether the decrease in the orbital period has been sudden or gradual. The radii of the components have been computed from the primary minimum alone. With the available spectroscopic data, the absolute dimensions of the components are also presented. It appears to be difficult to explain the evolution of the system without taking into consideration the mass loss. The proposed models for the evolution of the system and an explanation of the observed light variations are also presented.     

Light curve variations of RT lacertae

The eclipsing variable RT Lac, which is classified as an RS CVn-type binary by Hall, was observed in two colours,B andV, during the summer seasons of 1978 and 1979. The observations made during two successive seasons indicate that, outside eclipses, the system has brightened by about 0.12 and 0.15 magnitudes on average inB andV, respectively. Moreover, RT Lac was even fainter when observed by Milone in 1965. While the total brightness of the system remained unchanged at mid-secondary, it increased at mid-primary and outside eclipses. This may be interpreted as indicating that the cooler component is responsible for the peculiar light variations.     

Light curve variations in ER vulpeculae

Photoelectric observations of the peculiar eclipsing variable ER Vul were obtained in blue and yellow light, in the 1981 and 1982 observing seasons. The light curves suffer to change in short time-intervals. The wave-like distortion superimposed on the light curves is clearly seen, but sometimes there is no indication about its existence. The migration period has been estimated roughtly about eight months. Moreover, small-amplitude light fluctuations in the light curves are noticeable. These variations seem to be occur randomly. When the IUE and optical observations are taken into consideration together it is strongly suggested that both of the components in the system ER Vul are too active.     

Probing small scale structure in the atmosphere of V471 Tauri

The white dwarf in the eclipsing binary system V471 Tau is viewed through the atmosphere of the active K star prior to ingress and after egress. In the far UV the surface brightness of the hot white dwarf far outshines the K star emission. We can use this to probe the structure of the extended K star atmosphere along one line of sight, in absorption, on spatial scales of the radius of the white dwarf (10,000 km). The time series of HST/STIS spectra which show a hot (>250,000 K) extended (>1 K star radius) atmosphere around the K star. We see discrete structures in the velocity‐resolved spectra, on spatial scales of less than 100,000 km. The mean velocity is that expected of gas in co‐rotation with the K star, but the discrete velocity structures have excursions of up to 70 km/s from the mean. The mean temperature seems to increase with height above the K star photosphere. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New findings based on long-term photometric observations of the eclipsing binary V471 Tauri

The post-common envelope and pre-cataclysmic binary V471 Tau has been observed by the authors since 1973. At least a complete light curve in B and V bands and more than two eclipse timings were obtained in each year. All the available data published so far (including the authors') have been collected and analysed for the brightness and orbital period changes. The system brightened about 0.22 mag in both B and V bands more or less regularly up to 1997 and started to decrease afterwards. A search for periodicity of this variation yields a period longer than 85 yr. In addition to this long-period variation, a small amplitude of about 0.08 mag and short time-interval fluctuations on the mean brightness have been detected. The variations of the mean brightness have been discussed and plausible causes suggested. The changes of the apparent period have been attributed to a third body. Analysis of all the 'observed−calculated' (O−C) data yields a period of 32.4 yr, with a semi-amplitude of 151 s and an eccentricity of 0.30 for the third-body orbit. For orbital inclinations greater than 34° the mass of the third body would possibly match to a brown dwarf. One of the most interesting features in the light curve of V471 Tau is the decrement of the eclipse depth with time. The depth of the eclipse in the B band has been decreased from 0.082 to 0.057 mag over 34 yr. Subtracting the variation of the depth due to the brightening of the red dwarf star, the actual variation in depth, originated from from the white dwarf, was found to be about 0.012 mag. This change in the brightness of the compact object has been attributed to the mass accretion from its primary component via thermally driven wind and/or flare-like events.     

The ellipsoidal variables IV: Light variations

The V variations of ellipsoidal variables are investigated as a function of the spectral type of the primary. A model is developed on the assumption that the primary has near Main-Sequence characteristics. It is argued that the observed minimum in the V variations around spectral type F is real and is due to variations in the gravity-darkening exponent . The absolute upper limit to V variations is found to be 0.2 mag.     

Short-term brightness variations of V747 Cyg

We have conducted high-speed photometric observations of the suspected cataclysmic variable V747 Cyg. The literature data about the spectra of this star are conflicting. Zwitter and Munari [A&AS 107 (1994) 503] obtained spectroscopic observations which confirmed the cataclysmic variable nature of V747 Cyg, whereas Downes et al. [AJ 110 (1995) 1824] have concluded that this object may be a Be + M binary rather than a cataclysmic variable. Our observations revealed the complete absence of the rapid flickering in this star at a millimagnitude level. This phenomenon is to be inherent in all cataclysmic variables. Instead, the lightcurves of V747 Cyg showed probably periodic smooth low-amplitude variations that were visible during each observational night. When V747 Cyg was observed in two colours simultaneously, these variations had the equal phases and amplitudes. A periodogram analysis revealed probable periods of 5.77, 7.41 and 7.59 h. These periods and the amplitude as well as the oscillation behaviour in the different colours are typical of variable Be stars. Hence, V747 Cyg may be a variable Be star but not a cataclysmic variable.