The assessment of the near infrared identification of carbon stars - I. The local group galaxies WLM, IC 10 and NGC 6822

Context. The selection of AGB C and M stars from NIR colours has been done in recent years using adjustable criteria that are in needs of standardization if one wants to compare, in a coherent manner, properties of various populations. Aims. We intend to assess the NIR colour technique to identify C and M stars. Methods. We compare the NIR colours of several C stars previously identified from spectroscopy or narrow band techniques in WLM, IC 10 and NGC 6822. Results. We demonstrate that very few M stars have (J-K)(0) > 1.4 but a non negligible number of C stars are bluer than this limit. Thus, counts of M and C stars based on such limit do not produce pure samples. Conclusions. C/M ratios determined from NIR colours must be regarded as underestimates mainly because the M numbers include many warm C stars and also K stars if no blue limit is considered.