The brightness variations and orbital period changes of RT Lacertae

The light curves of the chromospherically active eclipsing binary RT Lacert ae obtained from 1993 to 1999 are analyzed here. The variation of the brigh tness at mid-eclipses and at maxima is carefully re-examined. The largest v ariation was obtained at mid-primary, where the more massive, hotter compon ent occults the less massive cooler secondary star. Therefore, we suggest t hat the variation of the system's brightness mainly arises from the more ma ssive star. The mean brightness of the system indicates a cyclic change. It showed at least two jumps during the last 22 years. The first occurred in 1984 and the second in 1994. Therefore, the length of the magnetic cycle ap pears to be about ten years. All the timings of the mid-eclipses obtained s o far were collected and analyzed under the assumption of the third body hy pothesis. A period of 94 yr was found for the third body orbit. The variati on of the systemic velocity of the eclipsing pair seems to confirm this sug gestion. The time delay and advance due to the orbit of the eclipsing pair around the third component were computed and subtracted from the original r esiduals obtained with the linear light elements. The remaining residuals a lso show a quasi-periodic change. The period of this change was calculated to be about 18 yr. This second O-C change may be related to the magnetic ac tivity of the more massive component.