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Deriving the distribution of binary parameters for a particular class ofstars over the full range of orbital separations usually requires thecombination of results from many different observing techniques (radialvelocities, interferometry, astrometry, photometry, direct imaging), each withselection biases. However, Cepheids---cool, evolved stars of $\sim$$5\,M_\odot$---are a special case because ultraviolet spectra will immediatelyreveal any companion star hotter than early type A, {\it regardless of theorbital separation}. We have used {\it International Ultraviolet Explorer}(\IUE) UV spectra of a complete sample of all 76 Cepheids brighter than V=8 tocreate a list of {\it all 18} Cepheids with companions more massive than $2.0\,M_\odot$. Orbital periods of many of these binaries are available fromradial-velocity studies, or can be estimated for longer-period systems fromdetected velocity variability. In an imaging survey with the {\it Hubble SpaceTelescope} Wide Field Camera 3, we resolved three of the companions (those of$\eta$ Aql, S Nor, and V659 Cen), allowing us to make estimates of the periodsout to the long-period end of the distribution. Combining these separationswith orbital data in the literature, we derive an unbiased distribution ofbinary separations, orbital periods, and mass ratios. The distribution oforbital periods shows that the $5\, M_\odot$ binaries have systematicallyshorter periods than do $1\, M_\odot$ stars. Our data also suggest that thedistribution of mass ratios depends both on binary separation and systemmultiplicity. The distribution of mass ratios as a function of orbitalseparation, however, does not depend on whether a system is a binary or atriple.

Binary Cepheids: Separations and Mass Ratios in $5\,M_\odot$ Binaries