2008A&A...477..615S

Double-lined eclipsing binaries are a traditional tool to test the capability of the stellar evolutionary models. If such binaries show apsidal motion, it is also possible to check, in addition to their absolute dimensions, some aspects of their internal structure. In order to perform this additional test, we monitored the times of a minimum of three eclipsing binaries with accurate absolute dimensions. Approximately thirty new precise times of minimum light recorded with CCD photometers were obtained for three early-type eccentric-orbit eclipsing binaries OX Cas (P=2.49d, e=0.041), PV Cas (1.75d, 0.032), and CO Lac (1.54d, 0.029). O-C diagrams were analyzed by the Lacy's method using all reliable timings found in the literature, and the elements of apsidal motion were improved. On the other hand, stellar models computed for the precise observed masses of the three systems were used as theoretical tools to compare with the observed shift in the periastron position. We confirm very short periods of apsidal motion of approximately 38.2, 91.0, and 43.4 years for OX Cas, PV Cas, and CO Lac, respectively. The relativistic effects are negligible, being up to 6% of the total apsidal motion rate in all systems. The corresponding observed apsidal motion rates are in good agreement with the theoretical predictions, except for the case of PV Cas, whose components seem to be more mass concentrated than the models predict.