2MASSJ22560844+5954299: the newly discovered cataclysmic star with the deepest eclipse

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 mediumresolution spectral observations. Modelling these data al lowed 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 obtaine d 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∼ 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, Hα, 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 mai nly in a vertically extended hot-spot halo. The emission Hα 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.