The triple system AO Monocerotis

The variable star AO Mon is a relatively bright but seldom investigated early-type eccentric eclipsing binary. Thirty new eclipses were measured as a part of our long-term observational project or derived from previous measurements. Based on a new solution of the current O–C diagram, we found for the first time a rapid apsidal advance superimposed with a light-time effect caused by a third unseen body in the system. Their short periods are 33.8 years and 3.6 years for the apsidal motion and the third-body circular orbit, respectively. The observed internal structure constant was derived to be log k2,\,obs = -2.23, which is close to the theoretically expected value. The relativistic as well as the third-body effects on the apsidal advance are negligible, as they are only about 3% of the total apsidal motion rate