The Mass of the Cepheid V350 Sgr

V350 Sgr is a classical Cepheid suitable for mass determination. It has a hot companion which is prominent in the ultraviolet (UV) 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 Space Telescope Imaging Spectrograph on the Hubble Space Telescope in the 1320 to 1510 Å 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 s −1 . This provides a mass ratio of the Cepheid to the companion of 2.1. The UV energy distribution of the companion provides the mass of the companion, yielding a Cepheid mass of 5.2 ± 0.3 M ⊙ . This mass requires some combination of moderate main sequence core convective overshoot and rotation to match evolutionary tracks.