A TiO study of the dwarf nova IP Pegasi

Abstract

We present red spectra in the region ∼λ7000–8300 Å of the eclipsing dwarf nova IP Peg, with simultaneous narrow-band photometry centred at 7322 Å. We show that by placing a second star on the slit we can correct for the telluric absorption bands which have hitherto made the TiO features from the secondary star unusable. We use these TiO features to carry out a radial velocity study of the secondary star, and find this gives an improvement in the signal-to-noise ratio of a factor of 2 compared with using the Na i doublet. In contrast with previous results, we find no apparent ellipticity in the radial velocity curve. As a result we revise the semi-amplitude to K2 = 331.3±5.8 km s−1, and thus the primary and secondary star masses to 1.05+0.14-0.07 M⊙ and 0.33-0.05+0.14 M⊙ respectively. Although this is the lowest mass yet derived for the secondary star, it is still overmassive for its observed spectral type. However, the revised mass and radius bring IP Peg into line with other cataclysmic variables in the mass-radius-period relationships. By fitting the phase-resolved spectra around the TiO bands to a mean spectrum, we attempt to isolate the light curve of the secondary star. The resulting light curve has marked deviations from the expected ellipsoidal shape. The largest difference is at phase 0.5, and can be explained as an eclipse of the secondary star by the disc, indicating that the disc is optically thick when viewed at high inclination angles.