Examples of synthetic fits (continuous and dotted lines) to the observed spectra (dots) for typical AGB carbon stars in the region of some HF lines. Panel a) (left): black line is the best fit to the R9 HF line in TX Psc (log ϵ(F) = 4.7,ξ = 2.2 km s^-1 and Γ = 13 km s^-1); red line is a fit assuming a 0.45 dex larger F abundance to show the effect of a F abundance variation similar to the total error estimated on [F/H] (see text); and the dotted red line shows the corresponding synthetic spectrum adopting the previously wrong lower excitation energy for this line, χ = 0.48 eV. Panel b) (centre): black line the same as case a; red line is a synthetic spectrum adopting a lower macroturbulence parameter Γ = 11 km s^-1; and the dotted red line adopts a lower microturbulence, ξ = 1.8 km s^-1 than in case a, respectively. Panel c) (right): effect of our new CN line list (black line) compared with the older line list (red line) on the fit to the R16 HF line at λ ~ 2.2778μm in the carbon star UU Aur.