Apsidal motion in southern eccentric eclipsing binaries: V539 Ara, GG Lup, V526 Sgr and AO Vel

Several new times of minimum light have been observed photoelectricaly for the early-type double-lined eccentric eclipsing binaries V539 Ara ($P = 3\fd2, e = 0.05$), GG Lup ($1\fd9, 0.15$), V526 Sgr ($1\fd9, 0.22$) and AO Vel ($1\fd6, 0.07$). The $\rm O{-}C$ diagrams are analysed using all reliable timings found in the literature and improved values for the elements of the apsidal motion are computed. We find more precise, relatively short periods of apsidal motion of about 162, 102, 155 and 54 years for V539 Ara, GG Lup, V526 Sgr and AO Vel, respectively. The corresponding internal structure constants, log k2, are then found to be –2.33, –2.16, –2.36 and –2.24 under the assumption that the component stars rotate pseudosynchronously. The relativistic effects are negligible, being about 6–7% of the total apsidal motion rate in all systems. Using the light-time effect solution, for V539 Ara we have found a third component orbiting with a period of about 42 years, for AO Vel we confirmed the third body with an orbital period of 33 years