Apsidal motion in the early-type eclipsing binaries CW Cephei, Y Cygni and AG Persei*

Abstract

Several new times of minimum light have been observed photoelectrically for the early-type double-lined eclipsing binaries CW Cep, Y Cyg and AG Per. From these observations, as well as data retrieved from the literature, new improved values of the apsidal periods of 45.5, 47.6 and 76.0 yr have been found, together with orbital eccentricities of 0.029, 0.146 and 0.070 for CW Cep, Y Cyg and AG Per, respectively. The corresponding internal-structure constants, log k2, are then found to be –2.08, –2.05 and –2.27 under the assumption that the component stars rotate pseudosynchronously, that is with the same angular velocity as the maximum orbital value. The results for each of the systems are discussed in the light of available absolute dimensions and theoretical models for the stellar density concentration. When compared to evolutionary tracks by Hejlesen, a general tendency is evidenced for a higher observed degree of internal density concentration than theoretically predicted, despite the consideration of the effect of evolution on log k2, while a much better agreement is found with the models computed by Jeffery. This confirms previous results obtained from the analysis of relatively massive binaries.