On the nature of SW Sex

We present spectrophotometry of the eclipsing nova-like variable SW Sex. Thecontinuum is deeply eclipsed and shows asymmetries due to the presence of abright spot. We derive a new ephemeris and, by measuring the eclipse width, weare able to constrain the inclination to i > 75^o and the disc radius to R_D >0.6 L_1. In common with other members of its class (of which it is theproto-type), SW Sex shows single-peaked emission lines which show transientabsorption features and large phase shifts in their radial velocity curves. Inaddition, the light curves of the emission lines show a reduction in fluxaround phase 0.5 and asymmetric eclipse profiles which are not as deep as thecontinuum eclipse. Using Doppler tomography, we find that most of the lineemission in SW Sex appears to originate from three sources: the secondary star,the accretion disc and an extended bright spot. The detection of the red starallows us to constrain the radial-velocity semi-amplitude of the secondary toK_R > 180 km/s and hence the component masses to M_1 = 0.3-0.7 Msun and M_2 <0.3 Msun.Comment: 15 pages, LaTeX, 10 figures, accepted by MNRAS, 19 June 199