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NGC 7789 is an intermediate-age open cluster with an age that is similar tothe mean age of contact binary stars. V12 is a bright UMa-type binary star withan orbital period of 0.397 day. The first complete light curves of V12 in V, R,and I bands are presented and analyzed with the W-D (Wilson and Devinney)method. The results shows that V12 is an intermediate-contact binary (f = 43%)with a mass ratio of 3.848, and it is a W-type contact binary where the lessmassive component is slightly hotter than the more massive one. The asymmetriesof the light curves are explained by the presence of a dark spot on the moremassive component. The derived orbital inclination (i = 83.6o) indicates thatit is a total eclipsing binary, which suggests that the determined parametersare reliable. The orbital period may show a long-term increase at a rate of P =+2.48 x 10-6 days/year that reveals a rapid mass transfer from the less massivecomponent to the more massive one. However, more observations are needed toconfirm this conclusion. The presence of an intermediate-contact binary in anintermediate-age open cluster may suggest that some contact binaries have avery short pre-contact timescale. The presence of a third body or/and stellarcollision may help to shorten the pre-contact evolution.

Photometric Investigation of the Total-Eclipsing Contact Binary V12 in the Intermediate-age Open Cluster NGC 7789