CLASSICAL CEPHEID MASSES - U-AQUILAE

We have obtained medium-resolution spectra (lambda/Delta lambda simila r to 20,000) of the hot binary companion to the classical Cepheid U Aq l with the Goddard High Resolution Spectrograph on the Hubble Space Te lescope (HST). These have been used to determine the orbital velocity amplitude. Combining this with the orbital velocity amplitude of the C epheid from the ground-based orbit and the mass of the companion infer red from its spectral type, we measure a mass of the Cepheid of 5.1 +/ - 0.7 M.. We discuss the full sample of Cepheids for which we have det ermined masses with HST (S Mus, V350 Sgr, Y Car, and U Aql) and also S U Cyg (mass from IUE). The HST masses are in agreement with the lumino sities predicted by recent evolutionary tracks with moderate overshoot . This comparison, however, may be altered by reassessment of Cepheid distances based on Hipparcos parallaxes.