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 physical parameters and to establish its peculiarities .

Methods : To obtain a light curve solution we used model whose emission sources are a white dwarf surrounded by an accretion disk with a hot spot , a gaseous stream near the disk ’ s lateral side , and a secondary star filling its Roche lobe .
The obtained physical parameters are compared with those of other SW Sex-subtype stars .

Results : The newly discovered cataclysmic 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 \sim 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 ^ { -1 } 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 .