Abstract

Binary population synthesis studies are powerful and extensively used tools to, e.g., reproduce the present-day population of close white dwarf binaries, or to predict the rate of Type Ia supernovae in the Galaxy. However, in these studies the common envelope (CE) phase is described by a simple parameterised energy equation in which a fraction of the orbital energy (the CE efficiency) is used to unbind the CE. Even worse, the CE efficiency is very uncertain and lacks observational constraints. During the last few years, our team has performed a large-scale observational population study of close white dwarf-main sequence binaries, which has provided the first strong observational evidence for the CE efficiency being small. However, we cannot rule out the possibility that this result is a consequence of selection effects, as white dwarfs can only be easily identified in the optical when the secondary is a low-mass star, i.e., an M dwarf. In order to test possible dependencies of the CE efficiency on the mass of the secondary star, we have initiated an observational campaign dedicated to identifying a large sample of close white dwarf-main sequence binaries containing F, G or K star companions and to measure their orbital periods. Here, I explain the methodology of our strategy and present the first results of this survey.