2002A&A...392..505S

We report the discovery of a new, bright (V∼17^m) AM Her system as the optical counterpart of the soft ROSAT All-Sky-Survey source RX J1846.9+5538 (=1RXS J184659.4+553834). Optical photometric and spectroscopic follow-up observations reveal a single period of 128.7min, consistent with a high degree of spin-orbit synchronization, and a low polar field strength (B<20MG) of the primary accretion region. The system was observed in optical intermediate and high states that differ by about 1 mag. These brightness variations were accompanied by a correlated change of the optical light curve, which we interpret as a switch between one- and two-pole accretion. This explanation is also supported by the X-ray light curves, which at two different epochs display emission from two equally bright accretion regions separated by ∼160°. Both spots possess distinct spectral X-ray properties as seen from the X-ray hardness ratio, where the secondary accretion region appears significantly softer, thus probably indicating a higher field strength compared to the primary region. In all ROSAT pointings a deep dip is present during the primary flux maxima, very likely caused by absorption in one of the accretion streams.