2006A&A...445.1037B

We report the first time-resolved photometric and spectroscopic optical observations of the X-ray source RXJ2133.7+5107, identified in the ROSAT survey. A clear persistent optical light pulsation is discovered with fast photometry at a period of P_ω=(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_Ω=(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, RXJ2133.7+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_ω/P_Ω 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)x10^–2M_☉ which, for a probable inclination i≤45° and a white dwarf mass M_wd=(0.6-1.0)M_☉, corresponds to a secondary mass M_s≥(0.27-0.37)M_☉.