Comparative abundance analysis of the hot main sequence stars and their progeny in open cluster M 25

Remarkable inconsistencies between elemental abundances in the main sequenc e stars and their progeny FG supergiants are discussed. Comparative abundan ce 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 t hese stars having a common origin but occupying at present different evolut ionary stages may be due to the fact that the chemical anomalies observed i n B stars are caused by the mechanism of the radiative diffusion in the upp er atmosphere layers. The chemical composition of B stars determined spectr oscopically 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 suffer ed the large scale mixing in the red giant phase. The observed abundances f or these objects are much more reliable as a reference point in the study o f 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.