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Aims: We calibrate spectroscopically the C- versus (vs.) M-type asymptoticgiant branch (AGB) star selection made using near-IR photometry, andinvestigate the spatial distribution of the C/M ratio in NGC 6822, based on lowresolution spectroscopy and near-IR photometry. Methods: We obtained lowresolution multi-object spectroscopy with the VIMOS instrument at the ESO VLTof ~ 800 stars in seven fields centred on NGC 6822. The spectroscopicclassification of giant stars in NGC 6822 and foreground dwarf contaminants wasmade by comparing more than 500 good quality spectra with the spectroscopicatlas of Turnshek et al. (1985). The sample of spectroscopically confirmed AGBstars in NGC 6822 is divided into C- and M-rich giants to constrain the C vs. MAGB star selection criteria based on photometry. The larger near-IR photometricsample is then used to investigate the C/M ratio gradients across the galaxy.Results: We present the largest catalogue of near-IR photometry and spectra ofAGB stars in NGC 6822 with 150 C-stars and 122 M-stars. Seventy-nine percent ofthe C-stars in our catalogue are redder than (J-K)0=1.2 mag, and 12% arebrighter than K0=16.45 mag and bluer than (J-K)0=1.2 mag. The remaining 9% aremixed with the M-type AGB stars, 88% of which have colours J-H0>0.73 mag and(J-K)0 between 0.9 mag and 1.2 mag. The remainder are mixed with dwarfs andC-type stars. The foreground dwarfs have preferably colours (J-H)0<0.73 mag(95%). Using the proposed criteria, we estimate that the overall C/M ratio ofthe galaxy is around 0.8 with a spread between 0.2<C/M<1.8. These resultssuggest that the metallicity index [Fe/H] is between -1.2 dex and -1.3 dexaccording to the different calibrations and that there is a significant spreadof about 0.4 - 0.6 dex. We also discuss age rather than metallicity variationsthat could explain the C/M ratio trends.

Spectra probing the number ratio of C- to M-type AGB stars in the NGC 6822 galaxy