Spectroscopy of the post-common envelope binary HW Virginis

We present optical spectroscopy of the post-common envelope binary HW Vir covering lambda = 3704 - 8667 Angstrom. We use the low-resolution blue spectra to determine the atmospheric parameters of the sdB primary star and the medium resolution H alpha observations to measure its radial velocity. We find K-1 = 82.3 +/- 4.0 km s(-1) and gamma = 2.9 +/- 3.1 km s(-1). The effective temperature, gravity and helium abundance of the sdB star are T-e = 28488 +/- 208 K, log g = 5.63 +/- 0.03 and N(He)/N(H) = 0.0066 +/- 0.0004. These characteristics are typical of a classical sdB star, as is our derived mass of M-1 = 0.48 +/- 0.09 M.. We find the distance to HW Vir is 171 +/- 19 pc. The predicted value of the projected orbital velocity of the secondary star is 284 +/- 21 km s(-1). Using previously published photometric light curves of HW Vir we corrected our spectra to absolute fluxes and extracted the spectrum of the secondary star. This spectrum shows H alpha absorption lines at phases close to the maximum of the reflection effect, with a measured radial velocity semi-amplitude of 275 +/- 15 km s(-1). We attribute these lines to irradiation of the face of the secondary star closest to the sdB star.