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As potential progenitors of several exotic phenomena including gravitationalwave sources, magnetic stars, and Be stars, close massive binary systems probea crucial area of the parameter space in massive star evolution. Despite theimportance of these systems, large uncertainties regarding the nature andefficiency of the internal mixing mechanisms still exist. In this work, we aimto provide robust observational constraints on the internal mixing processes byspectroscopically analyzing a sample of three massive overcontact binaries atdifferent metallicities. Using optical phase-resolved spectroscopic data, weperform an atmosphere analysis using more traditional 1D techniques and usingstate-of-the-art 3D techniques. We compare and contrast the assumptions andresults of each technique and investigate how the assumptions affect the finalderived atmospheric parameters. We find that in all three cases, bothcomponents of system are highly overluminous indicating either efficientinternal mixing of helium or previous non-conservative mass transfer. However,we do not find strong evidence of helium or CNO surface abundance changesusually associated with mixing. Additionally, we find that in unequal masssystems, the measured effective temperature and luminosity of the less massivecomponent places it very close to the more massive component on theHertzsprung-Russell diagram. These results were obtained independently usingboth of the techniques mentioned above, which suggests that these measurementsare robust. The observed discrepancies between the temperature and the surfaceabundance measurements when compared to theoretical expectations indicate thatunaccounted for additional physical mechanisms may be at play.

Constraining the Overcontact Phase in Massive Binary Evolution I. Mixing in V382 Cyg, VFTS 352, and OGLE SMC-SC10 108086