  Hot subdwarf stars represent a late and peculiar stage in the evolution of
low-mass stars, because they are likely formed by close binary interactions.
Here we performed a radial velocity (RV) variability study of a sample of 646
hot subdwarfs with multi-epoch radial velocities from Sloan Digital Sky Survey
(SDSS) and Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST)
spectra [...] As diagnostics we used the fraction of RV-variable stars and the
distribution of the maximum RV variations $\Delta RV_{\rm max}$. Both
indicators turned out to be quite inhomogeneous across the studied parameter
ranges. A striking feature is the completely different behaviour of He-poor and
He-rich hot subdwarfs. While the former have a high fraction of close binaries,
almost no significant RV variations could be detected for the latter. This led
us to the conclusion that there likely is no evolutionary connection between
these subtypes. Intermediate He-rich- and extreme He-rich sdOB/Os on the other
hand are likely related. We conclude further that the vast majority of this
population is formed via one or several binary merger channels. Hot subdwarfs
with temperatures cooler than $\sim24\,000\,{\rm K}$ tend to show less and
smaller RV-variations. These objects might constitute a new subpopulation of
binaries with longer periods and late-type or compact companions. The
RV-variability properties of the extreme horizontal branch (EHB) and
corresponding post-EHB populations of the He-poor hot subdwarfs match and
confirm the predicted evolutionary connection between them. Stars found below
the canonical EHB at somewhat higher surface gravities show large RV-variations
and a high RV-variability fraction, which is consistent with most of them being
low-mass EHB stars or progenitors of low-mass helium white dwarfs in close
binaries.
