The mass of the Cepheid binary V636 Scorpii

The mass-luminosity relation for Cepheids depends on the degree of mixing i n their main-sequence progenitors. Masses of Cepheids can be inferred by us ing different aspects of pulsation theory. These methods have in the past l ed to diverging results, showing that something was wrong in either evoluti on theory or pulsation theory or both. For some binary Cepheids dynamical m asses, which are independent of pulsation and evolution theories, can be de termined. V636 Sco is one of a handful of Cepheid binaries whose companions are bright enough in the ultraviolet that orbital radial velocities can be measured. We have here attempted to determine the radial velocity of the c ompanion V636 Sco B to the Cepheid V636 Sco A by means of two Hubble Space Telescope (HST) spectra taken at phases of minimum and maximum Cepheid orbi tal velocities. The ratio of the orbital velocity of the companion and the Cepheid (which is the inverse of their mass ratio) came out to be 1.25 +/- 0.17. V636 Sco B has a spectral type B9.5 V, for which we expect a stellar mass of 2.5 M-.. For the Cepheid we thus determine a mass of 3.1 +/- 0.4 Mo , which is surprisingly low. If true, such a low mass would indicate an ext remely large amount of excess mixing, which is very unlikely. We discuss the likely possibility that V636 Sco B may itself be a binary wi th an unseen secondary, in which case the mass derived from the two measure ments discussed above is not correct. Although the error limits for the mas s of each Cepheid are rather large, the combined mass-luminosity relation f or all Cepheids studied by us so far by means of HST spectra indicates exce ss mixing corresponding to core convective overshoot by 0.25 to about 0.5 p ressure scale height in the main-sequence progenitors of the Cepheids.