Abstract
We examined all of the galaxies in the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) to select those with a detected OIII λ4363 emission line, which allows to derive electron temperatures in HII 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 HeII λ4686 emission line. We confirm the increase of the N/O abundance ratio with decrease of the equivalent width EW(Hβ) of the Hβ 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.
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