We report on the discovery of variable circular polarization in the SW Sextantis star LS Pegasi. The observed modulation has an amplitude of ~0.3% and a period of 29.6 minutes, which we assume as the spin period of the magnetic white dwarf. We also detected periodic flaring in the blue wing of Hβ, with a period of 33.5 minutes. The difference between both frequencies is just the orbital frequency, so we relate the 33.5 minute modulation to the beat between the orbital 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's magnetosphere, which extends to the corotation radius. From this geometry, we estimate a magnetic field strength of B1~5-15 MG. Our results indicate that magnetic accretion plays an important role in SW Sex stars, and we suggest that these systems are probably intermediate polars with the highest mass accretion rates. Based on observations made with the William Herschel Telescope, operated on the island of La Palma by PPARC in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofísica de Canarias.