  We find a strong correlation between the optical outburst amplitude Delta V
and orbital period P_{orb} for the soft X-ray transient sources with orbital
periods less than 1 day. By fitting the observed values for 8 X-ray transients
we determine an empirical relation that can be used to predict the orbital
period of an X-ray transient given only its outburst amplitude: \Delta V= 14.36
- 7.63\log P_{orb}(h). For periods less than 12 hrs we determine a relation for
the absolute magnitude of the accretion disc during outburst, which then allows
us to estimate the distances to the sources.
