Introduction

Imaging, or mapping, is a natural part of our studies of binary stars, because of two factors: Stars spin on their rotation axes, usually in synchronism, and they orbit the centre of mass of the system, together with any other structures that belong to the binary. In effect, an observer ‘walks around’ the binary once per orbital period and is able to view the system from all of the orbital phase angles that are recorded via photometric, spectroscopic, and polarimetric means. In addition, because of the modem developments in astronomy, an observer has a more nearly bolometric view, and the radiation emitted from gases at very different temperatures, even within one binary system, can now be studied properly. Unfortunately, no close-binary system with interesting interactions between its components can yet be spatially resolved directly from Earth, because of the large distances to all the stars, so an observer sees only an integrated total amount of radiation at each observed wavelength from all the contributors in the binary – our points of light in the night-time sky again. But, as we have seen, the mutual eclipses of the components in suitably oriented binaries offer us a natural scanning mechanism that reveals the presence of those separate components, which can then be studied via all our observational tools. The Doppler effect, acting through the absorption/emission lines, provides velocities and hence inferred locations for the various contributors as well, and the polarization of the radiation reveals the distributions of scatterers and of magnetic fields.