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We present the N, O, F and Na abundance and 12C/13C isotopic ratiomeasurements 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-captureelements (CEMP-no). The abundances are derived from IR, K-band, high-resolutionCRIRES@VLT spectra obtained.  The metallicity of our sample ranges from [Fe/H]=-3.4 to -1.3. F abundancecould be measured only in two CEMP-s stars. With [F/Fe]=0.64, one is mildlyF-overabundant, while the other is F-rich, at [F/Fe]=1.44. For the remainingeight objects, including both CEMP-no in our sample, only upper limits on Fabundance could be placed. Our measurements and upper limits show that there isa spread in [F/C+N] ratio in CEMP-s stars as predicted by theory. Predictionsfrom nucleosynthetic models for low-mass, low-metallicity Asymptotic GiantBranch stars, account for the derived F abundances, while the upper limits on Fcontent derived for most of the stars are lower than the predicted values. Themeasured Na content is accounted for by AGB models in the 1.25 to 1.7 Msunrange, confirming that the stars responsible for the peculiar abundance patternobserved in CEMP-s stars are low-mass, low-metallicity AGB stars, in agreementwith the most accepted astrophysical scenario. We conclude that the mechanismof F production in current state-of-the-art low-metallicity low-mass AGB modelsneeds further scrutiny and that F measurements in a larger number of metal-poorstars are needed to better constraint the models.

Fluorine and Sodium in C-rich low-metallicity stars