Context : The masses previously obtained for the X-ray binary 2S 0921–630 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 ( v sin i ) 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 v sin i of the secondary star ( V395 Car ) in the low-mass X-ray binary 2S 0921–630 by comparison to an artificially broadened spectral-type template star .
In addition , we have also measured v sin i from a single high signal-to-noise ratio absorption line profile calculated using the method of Least-Squares Deconvolution ( LSD ) .

Results : We determine v sin i to lie between 31.3 \pm 0.5 km s ^ { -1 } to 34.7 \pm 0.5 km s ^ { -1 } ( assuming zero and continuum limb darkening , respectively ) in disagreement with previous results based on intermediate resolution spectroscopy obtained with the 3.6m NTT .
Using our revised v sin i value in combination with the secondary star ’ s radial velocity gives a binary mass ratio of 0.281 \pm 0.034 .
Furthermore , assuming a binary inclination angle of 75 ^ { \circ } gives a compact object mass of 1.37 \pm 0.13 M { { } _ { \odot } } .

Conclusions : We find that using relatively low-resolution spectroscopy can result in systemic uncertainties in the measured v sin i 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 v sin i measurement , we have revised down the compact object ’ s mass , such that it is now compatible with a canonical neutron star mass .