  This paper is part of a study of the apsidal motion in close eccentric
massive binary systems, which aims to constrain the internal structure of the
stars. We focus on the binary CPD-41{\deg}7742 and revisit HD152218.
Independent studies of CPD-41{\deg}7742 in the past showed large discrepancies
in the longitude of periastron of the orbit, hinting at the presence of apsidal
motion. We perform a consistent analysis of all observational data, explicitly
accounting for the apsidal motion. We use spectroscopic and photometric
observations of CPD-41{\deg}7742 to infer values for the fundamental stellar
and binary parameters. We apply a disentangling method to the spectra to
simultaneously derive the RVs at the times of observation and reconstruct the
individual stellar spectra. These are analysed by means of CMFGEN to determine
the stellar properties. We determine the apsidal motion rate in two ways: We
complement our RVs with those reported in the literature and we use the phase
shifts between the primary and secondary eclipses. The light curves are
analysed by means of Nightfall to constrain the orbital inclination and stellar
masses. Stellar structure and evolution models are constructed with Cles.
Different prescriptions for the mixing are adopted. Photometric data of
HD152218 are analysed and stellar evolution models are built. The binary
CPD-41{\deg}7742 displays apsidal motion at a rate of 15.38+0.42-0.51{\deg}
yr-1. Our analysis of the observational data of CPD-41{\deg}7742 explicitly
accounting for the apsidal motion allows us to explain the discrepancy in
periastron longitudes. The age estimates are in good agreement with estimates
obtained for other massive binaries in NGC 6231. This study confirms the need
for enhanced mixing in the stellar evolution models to reproduce the
observational stellar properties; this points towards larger convective cores
than usually considered.
