Spectroscopy of the double-lined dwarf nova AT Ara

The first time-resolved spectroscopic observations of the dwarf nova AT Ara are presented. The system is a double lined spectroscopic binary. Thus, radial velocity curves of both, emission lines from the accretion disk (in this particular case of H (alpha)) and of the secondary star can be measured. An orbital period of 0.37551 days is derived, placing AT Ara among the cataclysmic variables with the longest revolution periods. The RV-amplitudes yield a mass ratio of  q= M2/M1 = 079 +- 0.09. The spectral type of the secondary star is K2. AT Ara has an intermediate orbital inclination of  38(degree) +- 5(degree). The secondary star has a mass and a radius of M2 = 0.42 +- 0.10 M(circled dot operator) and  R2 = 0.84 +- 0.19 R(circled dot operator), respectively. It is thus significantly larger than a main sequence star of the same mass. Evolutionary calculations of cataclysmic variables suggest that the secondary is nuclearly evolved to a certain degree. The distance to AT Ara is estimated to be of the order of 600 pc.