XMM-Newton optical monitor observations of LMC X-3

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 similar to B5 subgiant filling its Roche lobe. Taking into account the Effect of X-ray irradiation, we suggest a value f(M) approximate to 1.5 M. 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.