Chemical fingerprints of binary mass transfer in massive stars

The majority of massive stars are born in close binary systems. As stars expand when they age, mass transfer or even a merger with their companion is inevitable. However, most binary interaction products appear as single stars, such that the main evidence of their exciting past is lost. Here, in a comprehensive grid of detailed massive binary evolution models, we find systematic trends in chemical surface abundances that allow the identification of the past mass gainers. We develop an analytic framework that is independent of specific evolutionary models, to constrain the amount and composition of the accreted material from their observed surface abundances. This yields tight constraints on the uncertain mass transfer physics in massive binary stars and allows us to reconstruct the past evolutionary history of the progenitor binary system. This method, which is shown to also constrain binary mergers (for example, SN 1987A), is applied to some of the best-studied OB stars so far. For γ Columbae, suggested to be an envelope-stripped star, we show that it is a mass gainer instead, whose companion star probably formed a stripped-envelope supernova. Our results highlight surface abundance measurements as a powerful tool to improve our understanding of massive binary systems evolving towards supernovae and compact object binaries.