  We estimate the fraction of F,G,K stars with close binary companions by
analysing multi-epoch stellar spectra from SDSS and LAMOST for radial velocity
(RV) variations. We employ a Bayesian method to infer the maximum likelihood of
the fraction of binary stars with orbital periods of 1000 days or shorter,
assuming a simple model distribution for a binary population with circular
orbits. The overall inferred fraction of stars with such a close binary
companion is 43.0% \pm 2.0% for a sample of F, G, K stars from SDSS SEGUE, and
30% \pm 8.0% in a similar sample from LAMOST. The apparent close binary
fraction decreases with the stellar effective temperature. We divide the SEGUE
and LEGUE data into three subsamples with different metallicity ([Fe/H] < -1.1;
-1.1 < [Fe/H] < -0.6; -0.6 < [Fe/H]), for which the inferred close binary
fractions are 56% \pm 5.0%, 56.0% \pm 3%, and 30% \pm 5.7%. The metal-rich
stars from our sample are therefore substantially less likely to possess a
close binary companion than otherwise similar stars drawn from metal-poor
populations. The different ages and formation environments of the Milky Way's
thin disk, thick disk and halo may contribute to explaining these observations.
Alternatively metallicity may have a significant effect on the formation and/or
evolution of binary stars.
