  Upsilon Sagittarii is a hydrogen-deficient binary that has been suggested to
be in its second stage of mass transfer, after the primary has expanded to
become a helium supergiant following core helium exhaustion. A tentative
identification of the faint companion in the ultraviolet led to mass estimates
of both components that made the helium star in Upsilon Sagittarii a
prototypical immediate progenitor of a type Ib/c supernova. However, no
consistent model for the complex spectrum has been achieved, casting doubt on
this interpretation. In the present study we provide for the first time a
composite spectral model that fits the ultraviolet data, and clearly identifies
the companion as a rapidly rotating slowly moving ~7Msun B-type star, unlike
previously suggested. The stripped helium supergiant is less luminous than
previous estimates, and, with an estimated mass smaller than 1Msun, is ruled
out as a core-collapse supernova progenitor. We provide a detailed binary
evolution scenario that explains the temperature and luminosity of the two
components as well as the very low gravity (log g ~ 1.0 [cm/s^2]) and extreme
hydrogen deficiency of the primary (atmospheric mass fraction XH ~ 0.001). The
best-fitting model is a ~5Msun intermediate-mass primary with an initial
orbital period of a few days, and a secondary that appears to have gained a
significant amount of mass despite its high rotation. We conclude that Upsilon
Sagittarii is a key system for testing binary evolution processes, especially
envelope stripping and mass accretion.
