AM Herculis star EV UMa (RE 1307+535) in a high accretion state

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 U Ma 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 l imit for magnetic CVs. During our observations EV UMa was in a very high ac cretion state, (15.7 less than or equal to V less than or equal to 17.2), w hich was about two magnitudes brighter than during our earlier (1994) high state observations(17.8 less than or equal to V less than or equal to 18.8) . Dramatic changes have occured in the circular polarization curves compare d to earlier observations, in particular the positive circular polarization is now reduced almost by factor of ten. Circular polarization varied durin g 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 fea ture is seen in the optical light curves between the orbital phases Phi = 0 .1 and Phi = 0.3. The presence of an eclipse and the reduction in the posit ive circular polarization during the same phase interval could be due to ef fects caused by the accretion stream. Our observations show that the accret ion rate has increased and the accretion geometry has changed in EV UMa sin ce 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) d ays.