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 supersoft X-ray sources was confirmed. The nature of the secondary star is discussed 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◦ ± 10◦. Doppler maps of the He iiλ4686 and Hα emission lines were constructed, which generally resemble 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.