  V350 Sgr is a classical Cepheid suitable for mass determination. It has a hot
companion which is prominent in the ultraviolet and which is not itself a
binary. We have obtained two high resolution echelle spectra of the companion
at orbital velocity maximum and minimum with the Hubble Space Telescope (HST)
Space Telescope Imaging Spectrograph (STIS) in the 1320 to 1510 \AA\/ region.
By cross-correlating these spectra we obtained the orbital velocity amplitude
of the companion with an uncertainty in the companion amplitude of 1.9 km
sec$^{-1}$. This provides a mass ratio of the Cepheid to the companion of 2.1.
The ultraviolet energy distribution of the companion provides the mass of the
companion, yielding a Cepheid mass of 5.2 $\pm$ 0.3 M$_\odot$. This mass
requires some combination of moderate main sequence core convective overshoot
and rotation to match evolutionary tracks.
