On the secondary star of the galactic supersoft X-ray source RX J0925.7-4758 and Doppler tomography of the emission lines

We carried out optical spectroscopic observations of the galactic supersoft X-ray source RX J0925.7-4758. The relatively long orbital period among sup ersoft X-ray sources was confirmed. The nature of the secondary star is dis cussed using its invisibility in the optical spectrum. Under the hypothesis of a non-eclipsing white dwarf, we derived an A-G giant secondary with an inclination angle of 55 degrees +/- 10 degrees. Doppler maps of the He II l ambda 4686 and H alpha emission lines were constructed, which generally res emble those of RX J0019.8+2156, suggesting a common line emitting region in supersoft X-ray sources. The concentration of the main emission region to low-velocities indicates that the mass function gives only a lower limit to the mass of the secondary star.