polar in the period gap

RXJ0501.7–0359 is a new V ∼ 17mag 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 ∼ 171min 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 = M2/M1 = 0.83 ± 0.15 and an inclination i = 75 ◦ ± 3 for the system. This yields a white dwarf mass M1 = 0.43 +0.10 −0.07 M⊙. Cyclotron humps might be present in some of our optical spectra leading to a tentative magnetic field strength of B ∼ 25MG.