2000A&A...355.1041G

We present new photometric and spectroscopic observations of the transient supersoft X-ray source RX J0537.7-7034 and find a periodicity of approximately 3.5 hrs. This establishes RX J0537.7-7034 as the supersoft X-ray source with the shortest orbital period. We furthermore derive an inclination of the binary system of 45°≲i≲70°, and the masses of the two binary components: M_accretor=0.6±0.2M_☉, M_donor=0.35±0.02M_☉. This implies that the standard scenario of supersoft X-ray sources, in which the donor is thought to be more massive than the accreting white dwarf to ensure high mass transfer rates on a thermal timescale (van den Heuvel et al. 1992), is not applicable for this system. We discuss alternative interpretations of this source as a former nova in which the thermonuclear flashes have become mild and most accreted mass is retained by the white dwarf (so-called SMC 13 type systems), or as a self-sustained wind-driven system.