THE ORBITAL PERIOD OF THE SU URSAE MAJORIS STAR EK TRIANGULI AUSTRALIS AND EVIDENCE FOR RING-LIKE ACCRETION DISKS IN LONG-SUPERCYCLE LENGTHSU URSAE MAJORIS STARS

An orbital period of 0.06288(5) d has been found from a radial velocit y study of the H alpha emission line. In addition, we have detected an extra line emitting source located approximate to 80 degrees apart fr om the vector joining the secondary-primary centers, as measured in th e opposite sense to the binary rotational motion. This is not the expe cted location for the hotspot in dwarf novae. This anomaly could be re moved by assuming a line emission lagging behind the white dwarf binar y motion. In addition, we have estimated line emissivity (proportional to r(-alpha)) and disk radius (R = r(in)/r(out)) for 8 SU UMa stars. Most stars fit alpha = 1.8 +/- 0.1 but AK Cnc and WZ Sge strongly devi ate from the mean; their emission line shapes can be explained assumin g a post-outburst accretion disk mostly emitting close to the white dw arf (AK Cnc) and a ring-like disk (WZ Sge). In addition, we have found a tendency of long-supercycle length SU UMa stars to show very compac t (large R; probably ring-like) accretion disks. If the supercycle len gth were basically controlled by the mass transfer rate ((M) over dot) , the inner disk radius would be a function of (M) over dot. A white d warf magnetic field similar to 5000 G is required to fit the truncatio n radius with the magnetosphere radius of SU UMa stars.