1997NewA....2..429M

There are now at least five sources classified as Soft X-ray Transients (SXTs) which show light curve modulations during their active states that are reminiscent of superhumps in SU UMa stars. We compare the morphology of the humps in SXTs with those in SU UMa stars and find that they are more analogous to the small subclass of ER UMa systems than to the bulk of the SU UMa's. The ER UMa stars apparently differ from other SU UMa systems only in having a much higher mass-transfer rate, one that is close to the stability limit of the thermal instability in the disc. In general SU UMa stars have very low-mass Roche-lobe filling components because a low mass ratio is required to excite the resonant instability, that probably drives the superhumps, in the outer edge of their accretion discs. If the degenerate components of SXTs are black holes rather than white dwarfs the system and hence the Roche-lobe filling component can be much more massive and still excite superhumps. Because X-ray irradiation of the disc by a central accreting neutron star or black hole stabilizes the thermal instability at lower mass-transfer rates, it is difficult to get transient behaviour in black-hole systems. Thus, mass-transfer rates in SXTs must naturally be close to the instability limit, and consequently these systems are similar to ER UMa stars in this sense. We test the determination of the mass, or at least a lower limit on it, of the compact primary of SXTs by means of the superhump period.