  We present the results of the study of the contact binary system BO CVn. We
have obtained physical parameters of the components based on combined analysis
of new, multi-color light curves and spectroscopic mass ratio. This is the
first time the latter has been determined for this object. We derived the
contact configuration for the system with a very high filling factor of about
88 percent. We were able to reproduce the observed light curve, namely the flat
bottom of the secondary minimum, only if a third light has been added into the
list of free parameters. The resulting third light contribution is significant,
about 20-24 percent, while the absolute parameters of components are: M1=1.16,
M2=0.39, R1=1.62 and R2=1.00 (in solar units). The O-C diagram shows an upward
parabola which, under the conservative mass transfer assumption, would
correspond to a mass transfer rate of dM/dt = 6.3 \times 10-8M\odot/yr, matter
being transferred from the less massive component to the more massive one. No
cyclic, short-period variations have been found in the O-C diagram (but
longer-term variations remain a possibility)
