THE MASS RATIOS OF CEPHEID BINARIES

The distribution of mass ratios in binary systems is important for comparison with star formation calculations but has been difficult to obtain observationally. This study uses IUE spectra of the hot companions of classical Cepheids with observed orbital motion to determine the companion types and hence their masses. Combining these with Cepheid masses 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 the Cepheid). This is a distribution of spectroscopic (as opposed to dynamic) mass ratios for intermediate-mass stars with orbital periods longer than a year. The distribution is strongly peaked to low-mass companions. The IUE spectra can detect companions in systems with mass ratios as small as 0.26. Incompleteness in detecting low-amplitude orbital motion means the concentration to low masses is even larger than that 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 components in the systems, except possibly in one case.