We study the optical counterpart of the black-hole X-ray binary LMC X - 3 , by using XMM-Newton/OM observations carried out during a low-hard X-ray state .
We derive a better constraint for the temperature , mass and radius of the companion star , and we show that the star is likely to be a \sim B5 subgiant filling its Roche lobe .
Taking into account the effect of X-ray irradiation , we suggest a value f _ { M } \approx 1.5 M _ { \odot } for the mass function in this system , lower than previously thought ; we provide a more accurate lower limit to the mass of the compact object .