New light curve analysis and period changes of the overcontact binary XY Leonis

Light curves and photometric solutions of the contact binary XY Leo are presented. The UBVR light curves appear to be almost symmetric. The B and V light curves were analysed with the Wilson-Devinney code and the orbital parameters of the system were obtained and compared with those obtained in previous studies. All the times of minimum light were collected and combined with the new ones obtained in this study. The orbital period of the system oscillates with a period of 19.6 years and a semi-amplitude of 0.023 day. This regular change was analysed under the assumption of the third body hypothesis. The time delay and advance due to the orbit of the eclipsing pair around the third component were computed and subtracted from the original residuals obtained with the linear light elements. The remaining residuals also show a parabolic change. Therefore, we assumed that the orbital period of the system changes with time due to at least two different causes. The parabolic change of the period may be attributed to either mass transfer from less massive component to the more massive one, or an additional component bounded to the double binary system with a longer period.