  This paper will review a new technique of detecting companion stars in LMXBs
and X-ray transients in outburst using the Bowen fluorescence lines at
4634-4640 Angs. These lines are very efficiently reprocessed in the atmospheres
of the companion stars, and thereby provide estimates of the K2 velocities and
mass functions. The method has been applied to Sco X-1, X1822-371 and GX339-4
which, in the latter case, provides the first dynamical evidence for the
presence of an accreting black hole. Preliminary results from a VLT campaign on
V801 Ara, V926 Sco and XTE J1814-338 are also presented.
