  NGC 7789 is an intermediate-age open cluster with an age that is similar to
the mean age of contact binary stars. V12 is a bright UMa-type binary star with
an 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 less
massive component is slightly hotter than the more massive one. The asymmetries
of the light curves are explained by the presence of a dark spot on the more
massive component. The derived orbital inclination (i = 83.6o) indicates that
it is a total eclipsing binary, which suggests that the determined parameters
are 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 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 or/and stellar
collision may help to shorten the pre-contact evolution.
