Evidence of Magnetic Accretion in an SW Sextantis Star: Discovery of Variable Circular Polarization in LS Pegasi

We report on the discovery of variable circular polarization in the SW Sexstar LS Pegasi. The observed modulation has an amplitude of ~0.3 % and a periodof 29.6 minutes, which we assume as the spin period of the magnetic whitedwarf. We also detected periodic flaring in the blue wing of Hbeta, with aperiod of 33.5 minutes. The difference between both frequencies is just theorbital frequency, so we relate the 33.5-min modulation to the beat between theorbital and spin period. We propose a new accretion scenario in SW Sex stars,based on the shock of the disk-overflown gas stream against the white dwarf'smagnetosphere, which extends to the corotation radius. From this geometry, weestimate a magnetic field strength of B(1) ~ 5-15 MG. Our results indicate thatmagnetic accretion plays an important role in SW Sex stars and we suggest thatthese systems are probably Intermediate Polars with the highest mass accretionrates.Comment: Accepted by ApJ Letters. LaTeX, 14 pages, 3 PostScript figure