It has long been recognized that tidal interaction between the components of close binaries will tend to circularize their orbits (e.g., Darwin 1879; Zahn 1977). In fact observations show an abundance of circular orbits in binaries with periods of days. However, the dissipation mechanism required for circularization, the timescales involved, and the dependence on period, mass, evolutionary state, and initial eccentricity of the system remain subjects of debate.
Circularization theories such as Zahn (1977) state that circularization in binaries where at least one component possesses a convective envelope is much more efficient than in binaries where both stars have radiative envelopes. Giuiricin et al. (1984; GMM) studied the period-eccentricity distribution of O-, B- and A-type binaries and argued that the observations were consistent with Zahn’s theory for radiative envelopes. But recently, Tassoul (1988) has presented an alternative circularization theory with similarly efficient circularization in radiative- and convective-envelope binaries. In addition, period-eccentricity distributions for many samples of main-sequence convective-envelope binaries are now available. Here we reconsider the observed short-period eccentricity distribution of radiative-envelope binaries in light of these new observations and theory. Specifically, we investigate the period-eccentricity distribution of a sample of binaries with A-type primary stars.