LUMINOSITY VARIATIONS AND ORBITAL PERIOD CHANGES IN RT LACERTAE

The chromospherically active binary RT Lac was observed between 1978 a nd 1992. These observations showed that the mean brightness of the sys tem varies with an amplitude of about 0.16m in blue and 0.11m in yello w light. The mean brightness at outside eclipses reached its maximum v alue at about 1986. During the observational time interval the mean co lour of the system also varied by 0.06m. The up-dated O-C values have been interpreted as the consequence of a light-time effect. The analys is indicates that the eclipsing pair revolves around a third body with a period of 80.71 yr. The projected distance of the eclipsing pair to the third body has been calculated to be approximately 176 times that of the separation between the eclipsing components. The lower limit e stimated for the mass of the third body is at least 1.77 solar masses which is slightly greater than the mass of the more massive companion. It turns out that the smaller the inclination is, the greater the mas s of the third body. If the inclination is decreased to 40-degrees the mass exceeds 3.5 M.. Due to the lack of a photometric evidence, the n ature of the third body should be questioned on the basis of stellar s tructure. When the light-time effect has been subtracted from all the photoelectric times of minima, an existence of cyclic variation in the orbital period revealed itself. The minimum of the O-C curve coincide s with the maximum light of the system. The period and the mean bright ness of the system seem to vary with the same cycle length of about 12 .06 yr. Since the light maximum coincides with the O-C curve's minimum , the outside spin of the more massive, active star should be greater than its inside spin according to the theory proposed by Applegate (19 92).