A TiO study of the dwarf nova IP Pegasi

We present red spectra in the region similar to lambda 7000-8300 Angstrom o f the eclipsing dwarf nova IP Peg, with simultaneous narrow-band photometry centred at 7322 Angstrom. We show that by placing a second star on the sli t we can correct for the telluric absorption bands which have hitherto made the TiO features from the secondary star unusable. We use these TiO featur es to carry out a radial velocity study of the secondary star, and find thi s gives an improvement in the signal-to-noise ratio of a factor of 2 compar ed 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 K-2 = 331.3 +/- 5.8 km s(-1), and thus the primary a nd secondary star masses to 1.05(-0.07)(+0.14) M. and 0.33(-0.05)(+0.14) M. respectively. Although this is the lowest mass yet derived for the seconda ry star, it is still overmassive for its observed spectral type. However, t he revised mass and radius bring IP Peg into line with other cataclysmic va riables in the mass-radius-period relationships. By fitting the phase-resolved spectra around the TiO bands to a mean spectr um, we attempt to isolate the light curve of the secondary star. The result ing 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 th ick when viewed at high inclination angles.