PHOTOMETRIC INVESTIGATION OF THE TOTALLY ECLIPSING CONTACT BINARY V12 IN THE INTERMEDIATE-AGE OPEN CLUSTER NGC 7789

NGC 7789 is an intermediate-age open cluster with an age similar to the mean age of contact binary stars. V12 is a bright W UMa-type binary star with an orbital period of 0.3917 days. The first complete light curves of V12 in the V , R , and I bands are presented and analyzed with the Wilson–Devinney (W-D) method. The results show that V12 is an intermediate-contact binary ( ) with a mass ratio of 3.848, and it is a W-type contact binary where the less massive component is slightly hotter than the more massive one. The asymmetry of the light curves is explained by the presence of a dark spot on the more massive component. The derived orbital inclination ( ) indicates that it is a totally eclipsing binary, which suggests that the determined parameters are reliable. The orbital period may show a long-term increase at a rate of days yr −1 that reveals a rapid mass transfer from the less massive component to the more massive one. However, more observations are needed to confirm this conclusion. The presence of an intermediate-contact binary in an intermediate-age open cluster may suggest that some contact binaries have a very short pre-contact timescale. The presence of a third body and/or stellar collision may help to shorten the pre-contact evolution.