  The HERMES spectrograph installed on the 1.2-m Mercator telescope has been
used to monitor the radial velocity of 13 low-metallicity carbon stars, among
which 7 Carbon-Enhanced Metal-Poor (CEMP) stars and 6 CH stars. All stars but
one show clear evidence for binarity. New orbits are obtained for 8 systems.
The sample covers an extended range in orbital periods, extending from 3.4 d
(for the dwarf carbon star HE 0024-2523) to about 54 yr (for the CH star HD 26,
the longest known among barium, CH and extrinsic S stars). Three systems
exhibit low-amplitude velocity variations with periods close to 1 yr
superimposed on a long-term trend. In the absence of an accurate photometric
monitoring of these systems, it is not clear yet whether these variations are
the signature of a very low-mass companion, or of regular envelope pulsations.
The period - eccentricity (P - e) diagram for the 40 low-metallicity carbon
stars with orbits now available shows no difference between CH and CEMP-s stars
(the latter corresponding to those CEMP stars enriched in s-process elements,
as are CH stars). We suggest that they must be considered as one and the same
family and that their different names only stem from historical reasons.
Indeed, these two families have as well very similar mass-function
distributions, corresponding to companions with masses in the range 0.5 - 0.7
Msun, indicative of white-dwarf companions, adopting 0.8 - 0.9 Msun for the
primary component. This result confirms that CH and CEMP-s stars obey the same
mass-transfer scenario as their higher-metallicity analogs, the barium stars.
The P - e diagrams of barium, CH and CEMP-s stars are indeed very similar. They
reveal two different groups of systems: one with short orbital periods (P <
1000 d) and mostly circular or almost circular orbits, and another with
longer-period and eccentric (e > 0.1) orbits.
