Radial Velocities of Visual and Interferometric Binaries

We discuss the use of precise radial velocities for orbit studies of binary stars. Although they are desirable to achieve highly accurate masses for systems of large amplitude, their greatest value will be for systems of long period and low amplitude. These studies require high spectrograph stability, and in some cases resolution of the spectra may prove troublesome. Radial velocities must be combined with visual or interferometric observations for mass determination, since eclipsing binaries of long period are fare, but the number of resolved spectroscopic systems is increasing. We review observational requirements, reduction techniques, and methods of solving for elements that satisfy simultaneously the observations in three dimensions, based on experience with radial velocities whose precision is near the loser limit for this Colloquium obtained over the past 35 years with the Dominion Astrophysical Observatory (DAO) coude spectrograph, and consider the gains that should be achievable over similar intervals in the future, by means of higher precision in all the relevant data. We discuss, from our own experience, the reasons why no masses have yet been determined by these methods, to an accuracy better than 2%.