The Role of the Sw Sextantis Stars in the Picture of Cv Evolution

The SW Sextantis stars are nova-like cataclysmic variables (CVs) which exhibit intricate behaviour which is still a matter of debate. For years the common belief has been that these systems were the " outlaws " of the CVs, mainly because of the lack of an overall understanding. We are now realising that a large percentage (∼ 30%) of all nova-likes in the ∼ 3 − 4 hour period range are SW Sex stars. The study of the dominance of this class just above the period gap will provide clues on the evolution of CVs. Here we present the discovery of 7 new SW Sex stars from the CVs found in the Hamburg Quasar Survey. 1. CV evolution: theory vs observations The currently accepted theory of CV evolution relies on the disrupted magnetic braking model. For P orb > 3 h angular momentum loss by stellar magnetic braking dominates, whereas emission of gravitational radiation is believed to be the only mechanism at P orb < 3 h. The observational input does not confirm a number of predictions of this theory, like the minimum period a CV can have (65 min; we observe ∼ 76 min) and a pileup of systems close to this minimum. To make things even worse, there is no observational evidence for a discontinous change in the spin-down rate due to magnetic braking between late-type field stars that are fully convective and those that have a radiative core (Andronov et al. 2003). The discrepancies between theory and observation are mainly due to strong selection effects. It is then fundamental to obtain an unbiased CV sample to compare its orbital period distribution with that predicted by theory, which will be a fundamental test. To do so, we have started a large scale search for new CVs in the Hamburg Quasar Survey (HQS; Hagen et al. 1995), where CVs were selected according to their spectroscopic fingerprint: broad emission or absorption lines, or a combination of the two. 3. The impact of SW Sex stars in the HQS sample About 20% of the 35 new HQS CVs for which we have measured P orb are SW Sex stars (Table 1). Striking is the concentration of these CVs within a nar