Nearly a quarter of all massive stars will merge with a companion during their lifetimes. This coalescence is preceded by a contact phase, which, while expected to be common, is poorly understood. This is due to a lack of observational constraints: less than ten O-type overcontact binaries are currently known. Recent theoretical studies have postulated that these massive overcontact binaries could be progenitors of several exotic classes of objects including magnetic massive stars, Be/Oe stars, LBVs and gravitational wave events. In this talk, I will discuss the current state of the field of massive overcontact binary research, with a specific focus on the chemically homogeneous evolution pathway. I will discuss the theoretical predictions as well as what the observational data tells us, and how these compare and contrast with one another. I will also describe a new spectroscopic analysis technique specifically designed to analyze these highly deformed systems and I will discuss how accounting for the 3D geometry can change our understanding of these objects.
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