  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.
