2009MNRAS.394L.136M

We present Very Large Telescope (VLT) intermediate resolution spectroscopy of UY Vol, the optical counterpart of the low mass X-ray binary (LMXB) X-ray burster EXO 0748-676. By using Doppler tomography we detect narrow components within the broad Heii λ4542, λ4686 and λ5412 emission lines. The phase, velocity and narrowness of these lines are consistent with their arising from the irradiated hemisphere of the donor star, as has been observed in a number of LMXBs. Under this assumption, we provide the first dynamical constraints on the stellar masses in this system. In particular, we measure K₂> K_em= 300±10 km/s. Using this value, we derive 1M_☉≤ M₁≤ 2.4M_☉ and 0.11 ≤ q ≤ 0.28. We find M₁≥ 1.5M_☉ for the case of a main sequence companion star. Our results are consistent with the presence of a massive neutron star as has been suggested by Ozel (2006), although we cannot discard the canonical value of ∼1.4M_☉.