1 Introduction

Cataclysmic variable stars (CVs) are interacting binaries consisting of a white dwarf accreting matter from a red dwarf donor. In non-magnetic CVs, the transferred gas spirals onto the white dwarf, forming an accretion disk. Due to the partial hydrogen ionization, the disk is thermally unstable, jumping in temperature when a certain critical density is reached. This hot state is accompanied by increased viscosity and a release of luminous energy when the material rapidly drops onto the white dwarf. This event is called an outburst. In some CVs, quasi-periodic humps are seen in the light curve during extended outbursts; they are called superhumps and the outbursts are called superoutbursts. According to current theories, these peculiar CVs, the so-called SU UMa stars, should contain low mass secondaries; some of them could have been eroded after a long time of mass transfer. It is even possible that many of them harbour secondary stars with masses less than the minimum mass needed to sustain hydrogen fusion in their cores (e.g. Howell et al. 2001). Such objects should be found among ultra-short orbital period systems ($P_{\rm o}$ $\sim$ 80 min) with very low mass transfer rates ( $\dot{M} \sim 10^{15}$ g/s), and should be characterized by rare and large-amplitude outbursts. This subgroup of the SU UMa stars is usually called the WZ Sge stars. Until now, a relatively small number of these objects have been studied in detail (Kato et al.  2001), in part due to the fact that their low luminosities make them hard to study in quiescence, even with large aperture telescopes. In this paper we make a contribution to the understanding of these rare objects presenting the first spectroscopic study of V 592 Herculis during superoutburst.