2009ApJ...696L.137W

We present a spectroscopic and photometric analysis of the remarkable massive binary system, BD+36°4063. We argue that the visible ON star is undergoing a rapid mass transfer episode that results in a thick and opaque disk that surrounds and renders invisible its massive companion. A comparison of the projected rotational velocity and the orbital semiamplitude of the visible star indicates a mass ratio near unity. Models for conservative mass transfer show that the equal mass state occurs at the point of minimum separation, and we argue that BD+36°4063 may represent the first system identified at this rapid and rare stage of massive binary evolution.