  We present the results of a Doppler tomographic reconstruction of the UV
spectra of the double-lined, O binary HD 100213 based on observations made with
the International Ultraviolet Explorer (IUE). We used cross-correlation methods
to obtain radial velocities, confirm the orbital elements, estimate the UV flux
ratio, and determine projected rotational velocities. The individual component
spectra are classified as O7 V + O8 V using UV criteria defined by Penny, Gies,
& Bagnuolo. We present a model fit of the eclipsing light curve from
observations from the Hipparcos satellite and published observations of
Andersen & Gronbech. We derive an orbital inclination, i=77.7deg+/-1.0deg. This
analysis indicates that both stars are currently experiencing Roche lobe
overflow (RLOF), which confirms earlier results that this is one of only a few
massive contact binaries. Our derived masses, Mp/Msolar=16.7+/-0.4 and
Ms/Msolar=10.4+/-0.4, are significantly lower than those computed from the
Doppler shifts of lines in the optical spectrum. We suggest that the difference
occurs because mutual irradiation decreases the upper atmospheric temperature
gradient in the inward-facing hemispheres of both stars, which makes lower
excitation lines appear weaker there and shifts their center of light away from
the center of mass. We compare the current state of HD 100213 with predicted
outcomes of massive close binary evolutionary models, and suggest that the
system is currently in a very slow case AA mass transfer stage.
