Introduction

The points of light in the night-time sky that we call stars can be divided into two categories. There are the truly single stars, like the Sun, which may happen to have a retinue of planets in orbit about them, with planetary masses that are found, at least in our Solar System, to total less than one-thousandth of the mass of the parent star. There are also pairs of stars, with the two components moving in bound orbits about their common centre of mass, which we call binary systems of stars, or just binary stars. Extensive observational programmes (Abt 1983) have demonstrated that single stars are about as common as binary stars, or, to put it another way, there are about 50% more individual stars in the sky than there are observable points of light. This means that the components of these binary stars are so close together that we cannot visually resolve them spatially into two separate stars. Appropriately, they are referred to as close binary stars, as distinguished from the more obvious visual binary stars, for which the observer can clearly resolve the two components and measure their apparent motions on the sky around the centre of mass of the binary system. Indeed, we have discovered a substantial number of visual binaries whose components are themselves close binaries, so that some apparently double, or even triple, stars have been found to be quadruple or sextuple systems.