  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.
