  We report an anticorrelation between continuum luminosity and the equivalent
width (EW) of the H-alpha emission line in X-ray binary systems. The effect is
evident both in a universal monotonic increase in H-alpha EW with time
following outbursts, as systems fade, and in a comparison between measured EWs
and contemporaneous X-ray measurements. The effect is most clear for black hole
binaries in the low/hard X-ray state, which is prevalent at X-ray luminosities
below ~1% Eddington. We do not find strong evidence for significant changes in
line profiles across accretion state changes, but this is hampered by a lack of
good data at such times. The observed anti-correlation, highly significant for
black hole binaries, is only marginally so for neutron star systems, for which
there are far less data. Comparison with previously established correlations
between optical and X-ray luminosity suggest that the line luminosity is
falling as the X-ray and optical luminosities drop, but not as fast
(approximately as L_{H-alpha} \propto L_X^{~0.4} \propto L_{opt}^{~0.7}). We
briefly discuss possible origins for such an effect, including the optical
depth, form of the irradiating spectrum and geometry of the accetion flow.
Further refinement of the relation in the future may allow measurements of
H-alpha EW to be used to estimate the luminosity of, and hence the distance to,
X-ray binary systems. Beyond this, further progress will require a better
sample of spectro-photometric data.
