RX J2133.7+5107: Identification of a new long period intermediate polar

We report the first time- resolved photometric and spectroscopic optical observations of the X-ray source RX J2133.7+5107, identified in the ROSAT survey. A clear persistent optical light pulsation is discovered with fast photometry at a period of P-omega = (570.823 +/- 0.013) s which we associate with the spin period of an accreting white dwarf. Radial velocity curves of the strong emission lines show modulation with a period of P-Omega = ( 7.193 +/- 0.016) h, identified as the orbital period. These observations establish that the source is a member of the intermediate polar class (IPs) of magnetic cataclysmic variables. With only 4 IPs with longer orbital periods, RX J2133.7+5107 is among the widest systems. It is a unique IP with an orbital period in the middle of the so-called (6- 10) h IP gap and it shows a significant degree of asynchronism with a ratio P-omega/P-Omega of 0.02. When attributed to the motion of the white dwarf, the emission lines orbital modulation yields a mass function of f(m) = (1.05 +/- 0.21) x 10(-2) M-circle dot which, for a probable inclination i <= 45 degrees and a white dwarf mass M-wd = (0.6-1.0) M-circle dot, corresponds to a secondary mass M-s >= (0.27-0.37) M-circle dot.