Long-term photometry of RT Lacertae: 1. Light and colour variations

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 li ght curves are presented. The light and colour variations in the four speci al regions of the light curve between 1978 and 1992 are discussed. The avai lable photometry obtained by the present authors defines at least three con secutive 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 c hanges in mid-primary and at second maximum harmonizes with the common prop erty of the RS CVn-type binaries. However, the light changes at first maxim um do not resemble either at second maximum or in mid-primary minimum. Ther efore, we conclude that the observed features cannot be attributed to the s pottedness 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 firs t maximum. The present data indicate that the magnetic cycle in RT Lac is a bout 12 years, which is analogous to the Sun's 11-year sunspot cycle.