THE MASS RATIOS OF CEPHEID BINARIES

The distribution of mass ratios in binary systems is important for com parison with star formation calculations but has been difficult to obt ain observationally. This study uses IUE spectra of the hot companions of classical Cepheids with observed orbital motion to determine the c ompanion types and hence their masses. Combining these with Cepheid ma sses inferred from an appropriate mass-luminosity relation produces a distribution of mass ratios q = M(2)/M(1) (where M(1) is the mass of t he Cepheid). This is a distribution of spectroscopic (as opposed to dy namic) mass ratios for intermediate-mass stars with orbital periods lo nger than a year. The distribution is strongly peaked to low-mass comp anions. The IUE spectra can detect companions in systems with mass rat ios as small as 0.26. Incompleteness in detecting low-amplitude orbita l motion means the concentration to low masses is even larger than tha t in the observed distribution. The fact that the orbital periods are longer than one year, and only one eccentricity is zero, implies that it is unlikely that there has been mass transfer between the component s in the systems, except possibly in one case.