Zeeman lines and a single cyclotron line in the low-accretion rate polar 1RXS J012851.9-233931 (RBS0206)

We present a low-resolution discovery spectrum and CCD-photometry of the br ight X-ray source 1RXS J0 12851.9-233931 found in the ROSAT All-Sky Survey. These first observations suggest that the source is an AM Herculis star (p olar) accreting at a low rate. The optical spectrum is dominated by Zeeman absorption features from the white dwarf, indicating a mean photospheric ma gnetic field of 36 +/- 1MG. Only weak Balmer line emission was observed. In the near infra-red, a single intense cyclotron hump was observed. The infe rred magnetic field strength in the accretion plama is 45 +/- 1 MG, the tem perature in the plasma is below 2keV. Likely orbital periods are similar to 90 min or similar to 146 min, the latter inside the cataclysmic variable p eriod gap. The system is an ideal target for further detailed investigation s of the field structure on a magnetic white dwarf by phase-resolved spectr opolarimetry.