  We present the N, O, F and Na abundance and 12C/13C isotopic ratio
measurements or upper limits for a sample of 10 C-rich, metal-poor giant stars,
eight enhanced in s-process (CEMP-s) elements and two poor in n-capture
elements (CEMP-no). The abundances are derived from IR, K-band, high-resolution
CRIRES@VLT spectra obtained.
  The metallicity of our sample ranges from [Fe/H]=-3.4 to -1.3. F abundance
could be measured only in two CEMP-s stars. With [F/Fe]=0.64, one is mildly
F-overabundant, while the other is F-rich, at [F/Fe]=1.44. For the remaining
eight objects, including both CEMP-no in our sample, only upper limits on F
abundance could be placed. Our measurements and upper limits show that there is
a spread in [F/C+N] ratio in CEMP-s stars as predicted by theory. Predictions
from nucleosynthetic models for low-mass, low-metallicity Asymptotic Giant
Branch stars, account for the derived F abundances, while the upper limits on F
content derived for most of the stars are lower than the predicted values. The
measured Na content is accounted for by AGB models in the 1.25 to 1.7 Msun
range, confirming that the stars responsible for the peculiar abundance pattern
observed in CEMP-s stars are low-mass, low-metallicity AGB stars, in agreement
with the most accepted astrophysical scenario. We conclude that the mechanism
of F production in current state-of-the-art low-metallicity low-mass AGB models
needs further scrutiny and that F measurements in a larger number of metal-poor
stars are needed to better constraint the models.
