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

RXJ 0501.7-0359 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 for the masses of the binary components and the orbital inclination. With an orbital period of P ~ 171 min the system is placed 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 = M_2/M_1 = 0.83 +/- 0.15 and an inclination i = 75 degr +/- 3 degr for the system. This yields a white dwarf mass M_1 = 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.