RX J0537.7-7034: The shortest-period supersoft X-ray source ?

We present new photometric and spectroscopic ob- servations of the transient supersoft X-ray source RX J0537.7- 7034 and find a periodicity of approximately 3.5 hrs. This es- tablishes RX J0537.7-7034 as the supersoft X-ray source with the shortest orbital period. We furthermore derive an inclina- tion of the binary system of 45 < i < 70, and the masses of the two binary components: Maccretor =0 :6 0:2 M, Mdonor =0 :35 0:02 M. This implies that the standard sce- nario 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 alter- native 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.