1 The Evolution of Binary Systems

One of the most important environments in which accretion discs are found occur in interacting binaries. In this chapter I review the main properties of binary sys-tems and the most important types of binary interactions, stable and unstable mass transfer, the role of mass loss, mass accretion and, in the most dramatic case, the merging of the two binary components. I particularly emphasise the evolutionary context in which these interactions occur and illustrate this using numerous ex-amples of different types of binaries of current research interest. These include hot subdwarfs, symbiotic binaries, binary supernova progenitors, including the progen-itors of Type Ia supernovae and potential progenitors of long-duration gamma-ray bursts, low-, intermediate- and high-mass X-ray binaries, containing both neutron stars and black holes, and their descendants, including binary millisecond pulsars, Thorne- ˙Zytkow objects and short-duration gamma-ray bursts. 1.