A spectroscopic binary in the Hercules dwarf spheroidal galaxy
  We present the radial velocity curve of a single-lined spectroscopic binary
in the faint Hercules dwarf spheroidal (dSph) galaxy, based on 34 individual
spectra covering more than two years of observations. This is the first time
that orbital elements could be derived for a binary in a dSph. The system
consists of a metal-poor red giant and a low-mass companion, possibly a white
dwarf, with a 135-days period in a moderately eccentric ($e=0.18$) orbit. Its
period and eccentricity are fully consistent with metal-poor binaries in the
Galactic halo, while the projected semimajor axis is small, at $a_p$ sin$i$ =
38 R$_{sun}$. In fact, a very close orbit could inhibit the production of
heavier elements through $s$-process nucleosynthesis, leading to the very low
abundances of neutron-capture elements that are found in this star. We discuss
the further implications for the chemical enrichment history of the Hercules
dSph, but find no compelling binary scenario that could reasonably explain the
full, peculiar abundance pattern of the Hercules dSph galaxy.
