The cessation of eclipses in SS Lacertae: The mystery solved

The 14.4 day period eclipsing binary SS Lacertae in the open cluster NGC 72 09 is unusual in that the eclipses stopped completely about the middle of t his century, giving rise to considerable speculation as to the cause. Disru ption of the binary through a collision with another member of the cluster and, more likely, gravitational perturbations from a third star in the syst em have been proposed to explain the phenomenon. We present here the result s of our intensive radial velocity monitoring of the object, which show cle arly that there is indeed a third star in the system in a slightly eccentri c orbit with a period of about 679 days. We also reanalyze the historical l ight curves of SS Lac to determine the properties of the system early in th e 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 r evealed apsidal motion in the system, (omega) over dot = 0.degrees 0137 cyc le(-1), corresponding to an apsidal period of about 1000 yr. We argue that the third star is probably responsible for this effect also.