M and S stars in the Magellanic Clouds

Digital spectra of 46 red stars in the field of the SMC and in globular clusters in both Magellanic Clouds reveal that 8 are K stars, 18 are M stars, 19 are early S stars and 1 is a foreground dwarf. Stars with B–V ∼ 2 in the field of the SMC may be of type K, M or S, though a common explanation for the abnormal colours in terms of an enhanced C/O ratio seems likely. Most of the non-carbon stars brighter than Mbol = −4.3 in the clusters are S stars, which evidently represent an intermediate stage in the modification of atmospheric composition. Systematic trends with cluster age are tentatively outlined, indicating that the M → S and S → C transitions occur at higher luminosity and lower surface temperature in the younger (more massive) stars.