  Aims: Study the high-mass X-ray binary X-7 in M33 using broad-band optical
data.
  Methods: We used recently published CFHT r' and i' data for variable stars in
M33 to extract the light curve of the optical counterpart of X-7. We combined
these data with DIRECT B and V measurements in order to search for an
independent optical modulation with the X-ray periodicity. The periodic
modulation is modelled with the ellipsoidal effect. We used UBVRr'i' magnitudes
of the system to constrain the temperature and radius of the optical component.
  Results: The optical data revealed a periodicity of 3.4530 +- 0.0014 days,
which is consistent with the known X-ray period. Double modulation, which we
attributed to ellipsoidal modulation, is clearly seen in four different optical
bands. The absolute magnitude in six optical bands is most consistent with a
stellar counterpart with 33000 < T_{eff} < 47000 K and 15 < R < 20 R_{\sun}. We
modelled the optical periodic modulation and derived the masses of the two
components as a function of the orbital inclination and the radius of the
stellar component. The resulting mass range for the compact object is 1.3 < M <
23 M_{\sun}.
  Conclusions: The system is probably a black hole HMXB, similar to Cyg X-1,
LMC X-1 and LMC X-3.
