  Hipparcos photometric data for the massive O-type binary UW CMa were analysed
within the framework of the Roche model. Photometric solutions were obtained
for five mass ratios in the $q=M_2/M_1=0.5-1.5$ range. The system is found to
be in a contact configuration. Independently of $q$, the best-fitting model
solutions correspond to the orbital inclination $i \sim 71^\circ$ and the
temperature of the secondary component $T_2 \sim 33500\,K$, at the fixed
temperature of the primary $T_1=33750 K$. Considering that the spectrum of the
secondary is very weak, photometric solutions corresponding to the contact
configuration favor the mass ratio $q$ smaller than unity (in which case the
luminosity of the secondary is smaller than that of the primary). The absolute
parameters of the system are estimated for different values of the mass ratio.
