The new long-period AM Herculis system RX J2157.5+0855

The identification and study of the optical counterpart of the ROSAT-discov ered AM Her system RX J2157.5+0855 is reported. We present high time-resolu tion spectrophotometric, spectropolarimetric, and multicolor CCD-photometri c observations of this polar obtained when the system was actively accretin g. The 3.375 hr orbital period of the system places it among a few long-per iod AM Her objects. The emission lines clearly reveal the presence of at le ast two different line components with different widths and radial velocity variations. The Balmer emission lines contain significant contribution fro m the X/UV-illuminated hemisphere of the secondary star. The Ha line is deb lended into two components, which we identify with the secondary star and t he accretion stream. The circular polarization unambiguously confirms the m agnetic nature of this newly discovered cataclysmic variable with approxima te to 20 MG strength deduced by cyclotron emission modeling. Combining radi al velocity and light curves with polarization variations provides addition al information about the system geometry.