  We compare Halpha emission profiles of 12 dynamically confirmed black holes
(BHs) and 2 neutron star X-ray transients (SXTs) in quiescence with those of a
sample of 43 Cataclysmic Variables (CVs), also quiescent. The full-width-half
maximum (FWHM) of the Halpha line in SXTs is tightly correlated with the
velocity semi-amplitude of the donor star K2=0.233(13) FWHM. This new
correlation, when combined with orbital periods (i.e. through photometric light
curves), opens the possibility to estimate compact object mass functions from
single integration, low-resolution spectroscopy. On the other hand, CVs above
the period gap are found to follow a flatter correlation, a likely consequence
of their larger mass ratios. We also find that the FWHM traces the disc
velocity at ~42% R_L1, independently of binary mass ratio. In addition, for a
given FWHM, BHs tend to have lower EWs than CVs. This might be explained by the
fact that CVs must be seen at higher inclinations to mimic the same projected
disc velocities as BH SXTs. For the same reason CVs with FWHM>~1500 km/s are
mostly eclipsing while none of our sample BHs are. Further, we show that there
is a vacant/unoccupied region for CVs in the FWHM-EW plane defined by FWHM >
2568 ( 1 - (9/EW)**2 )**0.5 (km/s). Both the FWHM-K2 correlation and the
FWHM-EW plane can be exploited, together with photometric light curves, to
efficiently discover quiescent BHs in deep Halpha surveys of the Galactic
Plane.
