  Context: The SW Sex stars are assumed to represent a distinguished stage in
CV evolution, making it especially important to study them. Aims: We discovered
a new cataclysmic star and carried out prolonged and precise photometric
observations, as well as medium-resolution spectral observations. Modelling
these data allowed us to determine the psysical parameters and to establish its
peculiarities. Results: The newly discovered vataclysmic variable
2MASSJ22560844+5954299 shows the deepest eclipse amongst the known nova-like
stars. It was reproduced by totally covering a very luminous accretion disk by
a red secondary component. The temperature distribution of the disk is flatter
than that of steady-state disk. The target is unusual with the combination of a
low mass ratio q~1.0 (considerably below the limit q=1.2 of stable mass
transfer of CVs) and an M-star secondary. The intensity of the observed three
emission lines, H_alpha, He 5875, and He 6678, sharply increases around phase
0.0, accompanied by a Doppler jump to the shorter wavelength. The absence of
eclipses of the emission lines and their single-peaked profiles means that they
originate mainly in a vertically extended hot-spot halo. The emission H_alpha
line reveals S-wave wavelength shifts with semi-amplitude of around 210 km/s
and phase lag of 0.03. Conclusions: The non-steady-state emission of the
luminous accretion disk of 2MASSJ22560844+5954299 was attributed to the low
viscosity of the disk matter caused by its unusually high temperature. The star
shows all spectral properties of an SW Sex variable apart from the 0.5 central
absorption.
