  We report the discovery of a new, bright (V = 17 mag) 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 160 degrees.
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.
