A new photometric method for identifying very distant carbon and late-M stars has been developed which utilizes two intermediate-band filters centered on CN and TiO absorption bands. The technique has been tested successfully on Galactic field stars and in eight regions in the disks of five nearby galaxies: M31, M33, NGC 6822, IC 1613, and WLM. The resulting carbon-to-M star ratios are directly related to parent-galaxy absolute magnitude, presumably reflecting a decreasing abundance in the lower-mass objects. The large C/M ratios observed in the Magellanic Clouds fit smoothly onto this relation, suggesting that this property of the Clouds is an internal one not much influenced by their location in the Milky Way's halo. In particular, the presence of a significant intermediate-age component may be a general characteristic of low-luminosity, late-type systems. A more complete survey of the asymptotic-giant-branch population in nearby galaxies should lead to considerable insight into the dependence of star-formation history on galaxy type. The potential use of the carbon-star luminosity function as a distance indicator is also discussed.
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Cook et al. (1986) studied this question.