The calibration of the metallicity versus C/M relation

This paper presents a critical reassessment of the C/ M ratio in the Local Group galaxies that have been surveyed by us in the last few years. We update distances, reddenings and metallicities with particular care to obtain coherent values for each galaxy. A new equation for the bolometric correction of M stars in terms of their ( R - I) colours is obtained from Bessell et al. ( 1998) data. We present a critical discussion on the colour - spectral type relation for M stars which reveals a dramatic lack of reliable data for late M stars. Finally, we show that - when homogeneous data are used - the log( C/ M0+) is well correlated to the [ Fe/ H] of the parent galaxy.