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We present the results of a spectroscopic monitoring campaign of the OB-starcompanions to the eclipsing X-ray pulsars SMC X-1, LMC X-4 and Cen X-3.High-resolution optical spectra obtained with UVES on the ESO Very LargeTelescope are used to determine the radial-velocity orbit of the OB(super)giants with high precision. The excellent quality of the spectraprovides the opportunity to measure the radial-velocity curve based onindividual lines, and to study the effect of possible distortions of the lineprofiles due to e.g. X-ray heating on the derived radial-velocity amplitude.Several spectral lines show intrinsic variations with orbital phase. Themagnitude of these variations depends on line strength, and thus provides acriterion to select lines that do not suffer from distortions. The undistortedlines show a larger radial-velocity amplitude than the distorted lines,consistent with model predictions. Application of our line-selection criteriaresults in a mean radial-velocity amplitude K(Opt) of 20.2 +/- 1.1, 35.1 +/-1.5, and 27.5 +/- 2.3 km/s (1 sigma errors), for the OB companion to SMC X-1,LMC X-4 and Cen X-3, respectively. Adding information on the projectedrotational velocity of the OB companion (derived from our spectra), theduration of X-ray eclipse and orbital parameters of the X-ray pulsar (obtainedfrom literature), we arrive at a neutron star mass of 1.06^{+0.11}_{-0.10},1.25^{+0.11}_{-0.10} and 1.34^{+0.16}_{-0.14} M{sun} for SMC X-1, LMC X-4 andCen X-3, respectively. The mass of SMC X-1 is near the minimum mass (~1 M{sun})expected for a neutron star produced in a supernova. We discuss theimplications of the measured mass distribution on the neutron-star formationmechanism, in relation to the evolutionary history of the massive binaries.

Determination of the mass of the neutron star in SMC X-1, LMC X-4 and Cen X-3 with VLT/UVES