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 \pm 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 \pm 0.016 ) hr , 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 ) hr 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 \pm 0.21 ) \times 10 ^ { -2 } M _ { \odot } which , for a probable inclination i \leq 45 ^ { \circ } and a white dwarf mass M _ { wd } = ( 0.6 - 1.0 ) M _ { \odot } , corresponds to a secondary mass M _ { s } \geq ( 0.27 - 0.37 ) M _ { \odot } .