Introduction

Of the three sub–types of interacting binary considered in this review, there is no doubt that classical novae (CN) are the best defined at present in terms of our knowledge of the composition of the central binary (white dwarf plus late-type main sequence star) and cause of outburst (thermonuclear runaway - see e.g. Bode and Evans 1989 and references therein). Recurrent novae (RN) on the other hand form a small, and surprisingly heterogeneous group of nine known members, with either red giant or main sequence mass–donating stars and either white dwarf or main–sequence accretors (see e.g. Bode 1987, Webbink et al. 1987). The much larger class of symbiotic stars (SS) is equally heterogeneous, and as with RN, the cause of outburst is less clear than for classical novae, though it seems that the evidence in favour of most of these systems containing a white dwarf is increasing (e.g. Mürset et al. 1991). What has been clear since the class was first defined is that their cool components are evolved.