2000A&A...361..189L

Remarkable inconsistencies between elemental abundances in the main sequence stars and their progeny F-G supergiants are discussed. Comparative abundance analysis of the hot main sequence stars, the cepheid U Sgr and two cool supergiants belonging to young open cluster M 25 is performed. The detected disaccord in the abundances of carbon, oxygen and other elements between these stars having a common origin but occupying at present different evolutionary stages may be due to the fact that the chemical anomalies observed in B stars are caused by the mechanism of the radiative diffusion in the upper atmosphere layers. The chemical composition of B stars determined spectroscopically may not reflect correctly their true chemical composition, nor the chemical composition of the interstellar medium. On the other hand such abundance anomalies are not expected for F-G supergiants which have suffered the large scale mixing in the red giant phase. The observed abundances for these objects are much more reliable as a reference point in the study of galactic chemical evolution. Three new Be stars are discovered in M 25. Our study has doubled the number of Be stars known in this cluster.