  A radial-velocity monitoring of the Carbon-Enhanced Metal-Poor (CEMP) star HE
0017+0055 over 8 years with the Nordic Optical Telescope and Mercator
telescopes reveals variability with a period of 384 d and amplitude of
540$\pm27$ m s$^{-1}$, superimposed on a nearly linear long-term decline of
$\sim$1 m s$^{-1}$ day$^{-1}$. High-resolution HERMES/Mercator and Keck/HIRES
spectra have been used to derive elemental abundances using 1-D LTE MARCS
models. A metallicity of [Fe/H] $\sim -2.4$ is found, along with s-process
overabundances on the order of 2 dex (with the exception of [Y/Fe] $\sim+0.5$),
and most notably overabundances of r-process elements like Sm, Eu, Dy, and Er
in the range 0.9 - 2.0 dex. With [Ba/Fe] $ > 1.9$ dex and [Eu/Fe] = 2.3 dex, HE
0017+0055 is a CEMP-rs star. It appears to be a giant star below the tip of the
red giant branch (RGB). The s-process pollution must therefore originate from
mass transfer from a companion formerly on the AGB, now a carbon-oxygen white
dwarf (WD). If the 384 d velocity variations are attributed to the WD
companion, its orbit must be seen almost face-on, with $i \sim 2.3^\circ$,
because the mass function is very small: $f(M_1,M_2) =
(6.1\pm1.1)\times10^{-6}$ Msun. Alternatively, the WD orbital motion could be
responsible for the long-term velocity variations, with a period of several
decades. The 384 d variations should then be attributed either to a low-mass
inner companion (perhaps a brown dwarf, depending on the orbital inclination),
or to stellar pulsations. The latter possibility is made likely by the fact
that similar low-amplitude velocity variations, with periods close to 1 yr,
have been reported for other CEMP stars in a companion paper (Jorissen et al.,
2015). A definite conclusion about the origin of the 384 d velocity variations
should however await the detection of synchronous low-amplitude photometric
variations.
