ИСТИНА

Photometric observations of the eclipsing variable star EX Dra acquired in the course of a program of studies of cataclysmic variables and their parameters recently carried out at the Sternberg Astronomical Institute (SAI) are presented. The star was observed with the 50-cm and 60-cm telescopes of the SAI Crimean Astronomical Station and a CCD photometer (~10500 images in the V and Rc filters, 24 observational nights) during the variable’s outburst and in a period of quiescence in 2014-2016. The orbital period was derived by the new numerous data in good agreement with the previous one. The light curves of EX Dra were plotted with this new value. Parameters of the system components (white and red dwarfs, accretion disk with hot spot and gaseous stream) were derived from 80 % of light curves using a “composite” model that takes into account the presence of a hot spot on the lateral surface of the geometrically thick disk and of a region of enhanced energy release near the disk edge, at the base of the gas flow (the so-called “hot line”). We also considered the variations of these parameters with the system transition from one stage of activity to another one. Six light curves of EX Dra were also used together with a “composite” model that additionally takes into account the presence of dark spots on the secondary component surface. The parameters of these dark spots on the red dwarf were determined. Two models were proposed to explain outbursts in dwarf novae: MTIM –mass transfer instability model and DIM – the termal-viscous disk instability model. Our data evidence that outbursts in EX Dra dwarf nova are due to the increase in the rate of accretion (MTIM model).