Context :

Aims : Study the high-mass X-ray binary X-7 in M33 using broad-band optical data .

Methods : We used recently published CFHT r ^ { \prime } and i ^ { \prime } 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 ^ { \prime } i ^ { \prime } magnitudes of the system to constrain the temperature and radius of the optical component .

Results : The optical data revealed a periodicity of 3.4530 \pm 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 .