2000A&A...357..677K

We present results of the first simultaneous UBVRI-photopolarimetry of the AM Herculis system EV UMa (RE 1307+535), carried out in February 1999. EV UMa is known to have a large variation in circular polarization (-20% to +50%) in the R-band. It has an orbital period close to the theoretical lower limit for magnetic CVs. During our observations EV UMa was in a very high accretion state, (15.7≤V≤17.2), which was about two magnitudes brighter than during our earlier (1994) high state observations (17.8≤V≤18.8). Dramatic changes have occured in the circular polarization curves compared to earlier observations, in particular the positive circular polarization is now reduced almost by factor of ten. Circular polarization varied during our observations from -23% to +5% in the U-band, from -15% to +6% in the B-, V-, and I-bands, and from -12% to 7% in the R-band. An eclipse-like feature is seen in the optical light curves between the orbital phases Φ=0.1 and Φ=0.3. The presence of an eclipse and the reduction in the positive circular polarization during the same phase interval could be due to effects caused by the accretion stream. Our observations show that the accretion rate has increased and the accretion geometry has changed in EV UMa since our previous observations. We have calculated an improved ephemeris for EV UMa, based on the negative circular polarization maxima observed in 1994 and 1999. The new value for the white dwarf spin period is 0.05533887(2) days.