2000A&A...354..579J

We performed time-resolved spectroscopic studies of the double-eclipsing dwarf nova EX Dra (formerly HS 1804+6753) in order to locate line emitting sites in the system. Optical spectra recorded during the quiescent as well as during the outburst state have been analysed by means of Doppler tomography. The computed Doppler images map the system in a variety of emission lines and allow us to compare between different temperatures and accretion states. Our studies revealed that the Balmer and HeI emission of EX Dra during quiescence is mainly formed within a fully established disk and within the gas stream. The Doppler map of H_α shows a second emission spot in the accretion disk located far from the region of interaction between the gas stream and the accretion disk. We have found a weak hint that secondary star emission contributes to the H_α line in quiescence, obviously caused by photospheric heating due to irradiation by the primary component. During outburst secondary star emission turns into a very strong emission source in the Balmer lines due to the increased accretion rate and an enhanced irradiation by the white dwarf or the boundary layer. The Doppler maps of the Balmer and HeI lines during outburst further show emission from the accretion disk. During outburst the gas stream is rarely seen in the Balmer lines but clearly visible in HeI and shows that the disk radius during this high accretion state is about 0.2R_L1 larger than during the recorded quiescent state. The origin of the CII (λ4267Å) line, which is only detectable during eruption can be located by Doppler imaging close to the primary component and may therefore be formed in the chromosphere of the white dwarf.