Relative abundance O/N versus C/N for several classes of stars (see text for references). Disk carbon stars (green starry points); post-AGB stars enriched in the s-process elements (blue filled triangles); hot post-AGB stars (blue filled squares); post-AGB stars not enriched in the s-process (blue open polygons); Population I supergiants (red plus sign); planetary nebulae (black open squares); HD 172481 (red filled square) and the stars analyzed in this work GLMP 334 and IRAS 15482 − 5741 (red triangles).