Spectra syntheses of the Na lines region. Top panel: spectra of the GIRAFFE AGB star AGB94048, bottom panel: spectra of the UVES RGB star RGB star RGB112452. The black asterisks represent the observed spectra; the red solid lines are the best-fit synthesised spectra; the blue dash-dotted lines and the green dashed lines are synthesised spectra, but with the best-fit Na abundance changed by plus/minus the error of Na abundance (± 0.07 dex for AGB94048 and ± 0.05 dex for RGB112452; here the error is a combination of random error and fitting error).