2009AJ....137.3700E

We have obtained spectra of the W Sgr system with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST). The spectra resolve the system into a distant companion B which is the hottest star in the system and the spectroscopic binary (A = Aa + Ab). A and B are separated by 0".16. We have extracted the spectra of both of these. We see no flux in the Aa + Ab spectrum which cannot be accounted for by the Cepheid, and put an upper limit on the spectral type and mass of the companion Ab of F5 V and ≤1.4 M_☉. Using the orbit from HST fine guidance sensor measurements from Benedict et al., this results in an upper limit to the mass of the Cepheid of ≤5.4 M_☉. We also discuss two possible distant companions. Based on the photometry from the Two Micron All Sky Survey Point Source Catalog, they are not physical companions of the W Sgr system.