Z. Tunca

Two-colour photoelectric investigation of SZ Pisces

The RS CVn-type eclipsing binary SZ Psc was observed in two coloursB andV, in 1981. The rate of change of the period was calculated using two different least-squares procedures. The amplitudes of the light variations at outside eclipses are 0.m181 and 0.m213 inB andV colours, respectively. The light curves of the system have been solved using a new approximation. The radius of the cooler component is only 82% of its Roche-radius and the system is detached one.

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 Ibanoğlu 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.

Two-colour photoelectric investigation of AR Lacertae

The RS CVn-type eclipsing binary AR Lac has been observed in two colours,B andV, in 1978 and 1979. Several times of minima have been obtained during the observations and the new light elements have been calculated. The orbital period of the system was found to be decreasing with an amount of 14.6 s century−1.The light curves of the system obtained inB andV show a significant wave-like distortion outside eclipses. The amplitudes of the wave-like distortion inB andV are about 0.m09 and 0.m10 in 1978, and, 0.m12 and 0.m13 in 1979, respectively. The minima of the wave-like distortion have shifted with an amount of 0.p40 in one year, thus indicating the migration period of the wave-like distortion is about 2.5 yr.A noticeable variation of light during the totality of primary minimum has also been observed. This variation can be attributed to the distortion wave which has been moving towards decreasing phases.

Photoelectric photometry of the peculiar system RT Lacertae

The observations of the peculiar eclipsing binary RT Lac were continued during the observing seasons of 1982 and 1983. The salient properties of the light curves were presented. The shallowest primary eclipse was observed in 1983. When the light curves, obtained in six years sucessively, are interpreted together it is seen that the variations of the brightness at mid-primary and second maximum are too similar to each other. The colour variations at mid-primary also follow those variations in the brightness. These circumstances indicate that the starspot hypothesis is insufficient alone to explain all the phenomena observed. The earlier spectral type G9 companion may be intrinsically variable and the unusual distortion on the light curves following the secondary eclipse may arise from the gas-stream from larger K star to the G9 star.

Photoelectric photometry of Z herculis

Two-colour light curves of Z Her were obtained observing the system from 1978 to 1981. The wave-like distortion at outside eclipses were derived. The amplitudes of the wave increase and its minimum migrates towards the decreasing orbital phase with a period of about 1.4 yr. An attempt was made to represent the wave-like distortions with a truncated Fourier series. The representation was found to be unsatisfactory since the distribution of the present observations is not regular but forms separate groups along the time axis. Therefore, further observations to cover the phases evenly in between the minima are needed.

Extensive photometric study of RT lacertae

In this paper we present the observations of the peculiar eclipsing binary RT Lac, which were obtained in 1980 and 1981. Examining the light curves obtained in successive four years we estimate the period of the migrating light curve distortion to be about four years. The orbital period of the system was also found to be decreasing with an amount 34s century−1. The brightnesses both at primary and secondary are changing with the time, which is clearly seen from the last light curves. The variations occurred at primary is three times greater than secondary eclipse. Therefore, we suggest that the star, in front at primary eclipse, is more responsible for the distorted light curves and it should be the earlier spectral type and also the smaller one.

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.

The new light curves and period study of AK Herculis

B andV observations of the W Ursae Majoris-type eclipsing variable system AK Her were made on five nights at the Ege University Observatory. Several times of minima were obtained during the observations and the new light elements were calculated. The light-time period was found to be about 75.72 years. The light curve of the system appears to change in each cycle for both colours. The secondary minimum of the system seems to be a total eclipse with a duration of about 42m.5.

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.

Long-Term Photometry of RT Lacertae: 1. Light and Colour Variation

The peculiar chromospherically active binary system RT Lac was observed in B and V bands between 1989 and 1992. The four-year photoelectric B and V light curves are presented. The light and colour variations in the four special regions of the light curve between 1978 and 1992 are discussed. The available photometry obtained by the present authors defines at least three consecutive maxima in the variation of the brightness of the system at second maximum and in mid-primary eclipse. The behaviour of the light and colour changes in mid-primary and at second maximum harmonizes with the common property of the RS CVn-type binaries. However, the light changes at first maximum do not resemble either at second maximum or in mid-primary minimum. Therefore, we conclude that the observed features cannot be attributed to the spottedness of the more massive component alone. The signs of the starspots are masked by the matter in front of the more massive component at the first maximum. The present data indicate that the magnetic cycle in RT Lac is about 12 years, which is analogous to the Suns 11-year sunspot cycle.