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Massive binary systems are important laboratories in which to probe theproperties of massive stars and stellar physics in general. In this context, weanalysed optical spectroscopy and photometry of the eccentric short-periodearly-type binary HD 152218 in the young open cluster NGC 6231. Wereconstructed the spectra of the individual stars using a separating code. Theindividual spectra were then compared with synthetic spectra obtained with theCMFGEN model atmosphere code. We furthermore analysed the light curve of thebinary and used it to constrain the orbital inclination and to derive absolutemasses of 19.8 +/- 1.5 and 15.0 +/- 1.1 solar masses. Combining radial velocitymeasurements from over 60 years, we show that the system displays apsidalmotion at a rate of (2.04^{+.23}_{-.24}) degree/year. Solving theClairaut-Radau equation, we used stellar evolution models, obtained with theCLES code, to compute the internal structure constants and to evaluate thetheoretically predicted rate of apsidal motion as a function of stellar age andprimary mass. In this way, we determine an age of 5.8 +/- 0.6 Myr for HD152218, which is towards the higher end of, but compatible with, the range ofages of the massive star population of NGC 6231 as determined from isochronefitting.

Apsidal motion in the massive binary HD152218