ON THE NATURE OF SW SEX

We present spectrophotometry of the eclipsing nova-like variable SW Se x. The continuum is deeply eclipsed and shows asymmetries as a result of the presence of a bright-spot. We derive a new ephemeris and, by me asuring the eclipse width, we are able to constrain the inclination to i > 75 degrees and the disc radius to R-D > 0.6L(1). In common with o ther members of its class (of which it is the prototype), SW Sex shows single-peaked emission lines which show transient absorption features and large phase shifts in their radial velocity curves. In addition, the light curves of the emission lines show a reduction in flux around phase 0.5 and asymmetric eclipse profiles which are not as deep as th e continuum eclipse. Using Doppler tomography, we find that most of th e line emission in SW Sex: appears to originate from three sources: th e secondary star? the accretion disc and an extended bright-spot. The detection of the red star allows us to constrain the radial velocity s emi-amplitude of the secondary to K-R > 180 km s(-1) and hence the com ponent masses to M-1 similar to 0.3-0.7 M. and M-2 < 0.3 M.. The Doppl er maps suggest a simple new model for SW Sex, in which the dominance of single-peaked line emission from the bright-spot over the weak doub le-peaked disc emission gives SW Sex its single-peaked profiles and fo rces the radial velocity curves to follow the motion of the bright-spo t and thus exhibit large phase shifts. The transient absorption featur es in the Balmer line profiles are mostly artefacts of the complex int ertwining of the emission components from the secondary star, bright-s pot and accretion disc and involve little true absorption. While the a ccretion disc and secondary star components of this model appear to be secure, the dominant bright-spot component fails in one important are a - its inconsistency with the Balmer line light curves. The eclipse p rofile requires the material emitting the Balmer lines to (a) be eclip sed as early as phase 0.8, (b) not be as deeply eclipsed as the contin uum, and (c) exhibit a Aat-bottomed eclipse and then come out of eclip se very sharply around phase 0.05. Although it is possible to explain the early ingress with a raised disc rim downstream from the bright-sp at, the rapid egress is difficult to account for without speculating e ither that there are regions of strong Balmer absorption in the disc, the changing visibility of which during eclipse alters the shape of th e light curve, or that there is Balmer emission from above the orbital plane which shares the velocity of the bright-spot.