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The role of mass and momentum exchanges in close massive binaries is veryimportant in the subsequent evolution of the components. Such exchanges produceseveral observational signatures such as asynchronous rotation and alteredchemical compositions, that remain after the stars detach again. Weinvestigated these effects for the close O-star binary LSS 3074 (O4 f + O6-7:(f):), which is a good candidate for a past Roche lobe overflow (RLOF) episodebecause of its very short orbital period, P = 2.185 days, and the luminosityclasses of both components. We determined a new orbital solution for thesystem. We studied the photometric light curves to determine the inclination ofthe orbit and Roche lobe filling factors of both stars. Using phase-resolvedspectroscopy, we performed the disentangling of the optical spectra of the twostars. We then analysed the reconstructed primary and secondary spectra withthe CMFGEN model atmosphere code to determine stellar parameters, such as theeffective temperatures and surface gravities, and to constrain the chemicalcomposition of the components. We confirm the apparent low stellar masses andradii reported in previous studies. We also find a strong overabundance innitrogen and a strong carbon and oxygen depletion in both primary and secondaryatmospheres, together with a strong enrichment in helium of the primary star.We propose several possible evolutionary pathways through a RLOF process toexplain the current parameters of the system. We confirm that the system isapparently in overcontact configuration and has lost a significant portion ofits mass to its surroundings. We suggest that some of the discrepancies betweenthe spectroscopic and photometric properties of LSS 3074 could stem from theimpact of a strong radiation pressure of the primary.

Observational signatures of past mass-exchange episodes in massive binaries: The case of LSS 3074