  CONTEXT: The masses previously obtained for the X-ray binary 2S0921-630
inferred a compact object that was either a high-mass neutron star or low-mass
black-hole, but used a previously published value for the rotational broadening
(vsini) with large uncertainties. AIMS: We aim to determine an accurate mass
for the compact object through an improved measurement of the secondary star's
projected equatorial rotational velocity. METHODS: We have used UVES echelle
spectroscopy to determine the vsini of the secondary star (V395 Car) in the
low-mass X-ray binary 2S0921-630 by comparison to an artificially broadened
spectral-type template star. In addition, we have also measured vsini from a
single high signal-to-noise ratio absorption line profile calculated using the
method of Least-Squares Deconvolution (LSD). RESULTS: We determine vsini to lie
between 31.3+/-0.5km/s to 34.7+/-0.5km/s (assuming zero and continuum limb
darkening, respectively) in disagreement with revious results based on
intermediate resolution spectroscopy obtained with the 3.6m NTT. Using our
revised vsini value in combination with the secondary star's radial velocity
gives a binary mass ratio of 0.281+/-0.034. Furthermore, assuming a binary
inclination angle of 75 degrees gives a compact object mass of 1.37+/-0.13Mo.
CONCLUSIONS: We find that using relatively low-resolution spectroscopy can
result in systemic uncertainties in the measured vsini values obtained using
standard methods. We suggest the use of LSD as a secondary, reliable check of
the results as LSD allows one to directly discern the shape of the absorption
line profile. In the light of the new vsini measurement, we have revised down
the compact object's mass, such that it is now compatible with a canonical
neutron star mass.
