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We present the radial velocity curve of a single-lined spectroscopic binaryin the faint Hercules dwarf spheroidal (dSph) galaxy, based on 34 individualspectra covering more than two years of observations. This is the first timethat orbital elements could be derived for a binary in a dSph. The systemconsists of a metal-poor red giant and a low-mass companion, possibly a whitedwarf, with a 135-days period in a moderately eccentric ($e=0.18$) orbit. Itsperiod and eccentricity are fully consistent with metal-poor binaries in theGalactic 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 ofheavier elements through $s$-process nucleosynthesis, leading to the very lowabundances of neutron-capture elements that are found in this star. We discussthe further implications for the chemical enrichment history of the HerculesdSph, but find no compelling binary scenario that could reasonably explain thefull, peculiar abundance pattern of the Hercules dSph galaxy.

A spectroscopic binary in the Hercules dwarf spheroidal galaxy