We report the discovery of a new , bright ( V \sim 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 128.7 min , consistent with a high degree of spin-orbit synchronization , and a low polar field strength ( B < 20 MG ) 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 \sim 160 ^ { \circ } .
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 .