Nitrogen abundance in the Blue Compact Dwarf galaxies from SDSS

We examined all of the galaxies in the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) to select those with a detected [O III] lambda 4363 emission line, which allows to derive electron temperatures in H it regions and element abundances by the direct method. We selected two sub-samples of galaxies: one with detected WR features in their spectra, and the other with the nebular He II lambda 4686 emission line. We confirm the increase of the N/O abundance ratio with decrease of the equivalent width EW(H beta) of the H beta emission line. This result is explained by gradual nitrogen enrichment of the interstellar medium by ejecta from massive stars in the most recent starburst episode.