Introduction

Measurement of the brightness of astronomical sources is a fundamental part of all astronomy. The first detections of variations in brightness levels led to the beginning of the recognition of the existence of eclipsing binary stars, thanks to the work of Goodricke (1783), as noted in Chapter 1. Since that time, systematic searches for photometrically variable stars have been conducted by visual, photographic, and photoelectric techniques, and many eclipsing, or photometrically variable, binary stars have been discovered in the process, again as discussed in general terms in Chapter 1. It is therefore no accident that the observational technique of photometry has been the mainstay of studies of binary stars in general, at least in part because photometry can be conducted with very modest equipment and relatively small telescopes. Recent developments in CCD technology that have made it possible to record digital images of small areas of the sky with remarkable photometric precision have further enhanced the value of telescopes of modest apertures in conducting front-line research – for example, the recent discoveries of substantial numbers of eclipsing binaries in the nearby galaxy M31 by Kaluzny et al. (1998, 1999) and Stanek et al. (1998, 1999), who used a telescope of 1.3-m aperture. The current prospects for setting up a global network of programmable robotic telescopes to conduct continuous photometric monitoring of variable sources, including binary stars, are very encouraging, and that will herald a new era in our understanding of the properties of binary stars – magnetic-activity cycles, outbursts, changes in accretion structures, for example.