A study of two post-common envelope binary systems

Abstract

We present intermediate-resolution, phase-resolved spectra of the close-binary systems EC 11575−1845 and V664 Cas, the central star of the planetary nebula HFG 1. Both systems have a very rich emission-line spectrum originating from the face of the cool star, which is irradiated by the hot subdwarf star. The H i emission lines are wide, consistent with Stark broadening. These have a complex shape due to strong absorption near the line centre. We report on new spectroscopic orbits for both subdwarf stars, based on radial velocity measurements of their He ii lines. The orbits of the heated face of the cool companions were measured from narrow emission lines, and from the wide H i profiles; some differences were found between ions. At phases around 0, absorption lines from the photosphere of the secondary star of V664 Cas are seen. These are used to estimate a spectral type of mid-F to early K, and we report on their radial velocities. Synthetic spectra from the non-local thermodynamic equilibrium model atmospheres were used to fit features in the optical spectrum of the hot subdwarf star of EC 11575−1845, from which we estimate Teff= 105 000 ± 20 000 K, log g= 6.7 ± 0.3, log nHe/nH=−0.8 ± 0.1 . Finally, we consider the most likely range of component masses for these two systems.