Carbon, nitrogen, oxygen and lithium abundances of six cool supergiants in the SMC

Carbon, nitrogen and oxygen abundances were derived from high-resolution spectra of 6 cool supergiants of the Small Magellanic Cloud. Oxygen-to-iron ratios (mean value <([O/Fe])over bar> approximate to-0.18 dex) are found to be similar to those found in young objects in the LMC and the Galaxy. This result is discussed in terms of chemical evolution. A mean deficiency of the carbon-to-iron ratio of <([C/Fe])over bar> approximate to-0.3 dex and a nitrogen-to-iron ratio <([N/Fe])over bar> approximate to+0.22 dex might imply a mixing signature, which is confirmed by the low C-12/C-13 ratio measured in three stars. In terms of chemical evolution, carbon plus nitrogen abundances have to be considered: in our sample, a mean value of <([(C + N)/Fe])over bar> -0.15 dex indicates a slight deficiency in carbon plus nitrogen with respect to the Sun, similar to the deficiencies found in Galactic supergiants and Orion. Lithium has been detected in all the program stars, reaching epsilon(Li)=0.6 dex for two stars.