Photometric study of the contact binary star V861 Herculis

First Cousins $V(RI)_{\rm C}$ light curves of the W UMa-type eclipsing variable star V861 Herculis are presented. The system was observed in 2000 and 2003. The light curves obtained at different epochs showed significant deviation from each other. The O'Connell effect was about $\Delta V = 0.05$ mag in 2000 but it vanished in 2003 (except the night of February 22/23, 2003 when we observed a small O'Connell effect ($\Delta V = 0.02$)). Further peculiarities of the light curves are also discussed. The light curves were solved with the Wilson-Devinney code and the spot parameters were determined. The system seems to be a W-type contact binary system with at least one active component. This is strongly supported by the fact that the system is brighter by 0.45 mag through an $\rm H\alpha$ filter than the MS stars with the same colour index in the open cluster NGC 7790. This deviation can be explained by chromospheric activity of one of the components.