2000AJ....119.1914T

The 14.4 day period eclipsing binary SS Lacertae in the open cluster NGC 7209 is unusual in that the eclipses stopped completely about the middle of this century, giving rise to considerable speculation as to the cause. Disruption of the binary through a collision with another member of the cluster and, more likely, gravitational perturbations from a third star in the system have been proposed to explain the phenomenon. We present here the results of our intensive radial velocity monitoring of the object, which show clearly that there is indeed a third star in the system in a slightly eccentric orbit with a period of about 679 days. We also reanalyze the historical light curves of SS Lac to determine the properties of the system early in the century. We discuss the implications of our findings in terms of changes in the inclination angle of the close pair. An analysis of available times of eclipse for SS Lac together with the new velocity information has also revealed apsidal motion in the system, ω{dot}=0.0137°.cycle^–1, corresponding to an apsidal period of about 1000 yr. We argue that the third star is probably responsible for this effect also.