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We report high-resolution optical spectroscopy of the low-mass X-ray binaryV395 Car/2S 0921-630 obtained with the MIKE echelle spectrograph on theMagellan-Clay telescope. Our spectra are obtained near superior conjunction ofthe mass donor star and we exploit the absorption lines originating from theback-side of the K-type object to accurately derive its rotational velocity.Using K0-K1 III templates, we find vsini=32.9 +/- 0.8 km/s. We show that thechoice of template star and the assumed limb darkening coefficient has littleimpact on the derived rotational velocity. This value is a significant revisiondownwards compared to previously published values. We derive new systemparameter constraints in the light of our much lower rotational velocity. Wefind M_1=1.44 +/- 0.10 Msun, M_2=0.35 +/- 0.03 Msun, and q=0.24 +/- 0.02 wherethe errors have been estimated through a Monte-Carlo simulation. A possibleremaining systematic effect is the fact that we may be over-estimating theorbital velocity of the mass donor due to irradiation effects. However, anycorrection for this effect will only reduce the compact object mass further,down to a minimum mass of M_1=1.05 +/- 0.08 Msun. There is thus strong evidencethat the compact object in this binary is a neutron star of rather typical massand that the previously reported mass values of 2-4Msun were too high due to anover-estimate of the rotational broadening.

On the mass of the neutron star in V395 Car/2S 0921-630