Period variation of six RS CVn-type binaries with possible light-time effect

Long term period variation of six RS CVn type binaries (RT And, SV Cam, WY Cnc, CG Cyg, WW Dra, and RT Lac) formed by using the observed times of eclipse minima are presented and discussed with regard to the possible causes. The light-time effect due to invisible component stars was found to be the most possible cause. In all cases, the observed times of eclipse minima do not cover more than one complete cycle in the periodic (O-C) diagrams. Thus, the validity of the light-time effect is not definite for our sample stars, although the resulting orbital parameters and masses of the hypothetical third components are physically acceptable