RX J0501.7-0359: a new ROSAT discovered eclipsing polar in the period gap

RX J0501.7-0359 is a new V 17 mag eclipsing polar discovered during the ROSAT all-sky survey (RASS). Extensive follow-up observations at X-ray (pointed ROSAT PSPC and HRI observations) and optical (time-resolved photometry and spectroscopy) wavelengths were obtained, which allowed us to determine a very precise orbital period and to provide constraints on the masses of the binary components and the orbital inclination. With an orbital period of P 171 min the system is at the upper end of the period gap. From the radial velocity amplitude of the secondary and the duration of the primary eclipse we obtain a mass ratio q = M₂/M₁ = 0.83 plus or minus 0.15 and an inclination i = 75 deg plus or minus 3 deg. This yields a white dwarf mass M₁ = 0.43^{+0.10}_{-0.07} {M}_sun. Cyclotron humps might be present in some of our optical spectra leading to a tentative magnetic field strength of B 25 MG.