On the outburst amplitude of the soft X-ray transients

We find a strong correlation between the optical outburst amplitude ΔV and orbital period P_orb for the soft X-ray transient sources with orbital periods less than 1 d. By fitting the observed values for eight 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:

ΔV=14.36−7.63 log P_orb(h).

For periods less than 12 h 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.