  Mounting evidence from subdwarf B (sdB) stars in the galactic field and their
recently discovered counterparts in old open clusters indicates that at least
two thirds of local disk sdB stars are binaries. Our recent radial velocity
survey showed that sdB binaries naturally divide into two groups with
contrasting spectroscopic and kinematic properties. Those with detectable
spectral lines from a cooler companion invariably have periods longer than a
year, while very short period sdB's have essentially invisible companions. We
derive typical orbital separations for the components of the composite spectrum
sdB's from their velocities. The current systems must have been produced by
Roche lobe overflow/mass transfer from low mass, metal-rich giants near the
first red giant branch tip, without undergoing common envelope envolution. The
same process should also occur at slightly lower red giant luminosities,
producing a wide binary with a helium white dwarf instead of an sdB star. Most
short period sdB's probably result from a common envelope following Roche lobe
overflow of the initial secondary onto the white dwarf. Rare post-common
envelope sdB + main sequence (MS) binaries also exist, but available data
suggest that most such systems involving lower MS companions end up merging.
The small and nearly identical masses of the two known MS survivors in short
period sdB binaries imply that both components must have lost a large fraction
of their initial mass in the common envelope process.
