THE MASS OF THE CLASSICAL CEPHEID V350 SAGITTARII

Two medium-resolution spectra of the hot companion of the Cepheid V350 Sgr have been obtained with the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope. Comparison of these spectra (in the wavelength range 1840-1880 Angstrom) with the spectrum of alpha Lyr s hows that V350 Sgr B has a projected rotation velocity which may be as high as 150 km s(-1). The velocity difference between the spectra of V350 Sgr B at two orbital phases is measured to be -23.1+/-3.8 km s(-1 ). The error is dominated by the aperture centering. When combined wit h the orbital velocity variation of the Cepheid derived from the groun d-based orbit and the mass of the companion deduced from IUE spectra, the mass of the Cepheid is found to be 5.2+/-0.9 M.. The observed mass -luminosity combination of V350 Sgr A is a good match to recent evolut ionary calculations which use moderate convective overshoot near the m ain sequence, however, the blue loops do not extend to temperatures as hot as the Cepheid.