The mass of the neutron star in the low – mass X – ray binary 2 A 1822 – 371 ∗

Using phase resolved spectroscopic observations obtained with the Ultraviolet and Visual Echelle Spectrograph on ESO’s Kueyen Very Large Telescope supplemented by spectroscopic observations obtained with the Boller and Chivens Spectrograph on the Walter Baade Magellan telescope, we found sinusoidal radial–velocity variations with a semi–amplitude 327±17 km/s. From previous observations and from the fact that the epoch of minimum velocity arrived early with respect to the epoch calculated from pulse timing we know that the companion star is suffering from irradiation. Since we most likely observed primarily the side of the companion star facing the observer at phase ∼0.75 the velocity quoted above is not the true radial velocity semi–amplitude of the companion star. Assuming a uniform contribution to the line profile from this hemisphere yields a radial velocity semi–amplitude of 280±26 km s for a systemic velocity of 54±24 km s; if the contribution is instead weighted somewhat more towards the side of the companion facing the X–ray source then the true semi–amplitude is larger than this value. Together with the well constrained inclination (81 < i < 84) and the mass–function determined from pulse–timing analysis (2.03±0.03×10M⊙), we derive a lower limit to the mass of the neutron star and to that of the companion star of 0.97±0.24M⊙ and 0.33±0.05M⊙, respectively (1σ; including uncertainties in the inclination). We briefly discuss other aspects of the spectrum and the implications of our findings.