  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) 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 +/- 0.21) 10^{-2} Msun which,
for a probable inclination i < 45 deg and a white dwarf mass M_{wd} = (0.6-1.0)
Msun, corresponds to a secondary mass M_{s} > (0.27-0.37) Msun.
