The chemical composition of the small magellanic cloud HII region NGC 346 and the primordial helium abundance

Spectrophotometry in the lambda lambda 3400-7400 range is presented for 13 areas of the brightest H II region in the SMC: NGC 346. The observations were obtained at CTIO with the 4 m telescope. Based on these observations, its chemical composition is derived. The helium and oxygen abundances by mass are given by Y(SMC) = 0.2405 + 0.0018 and O(SMC)= 0.00171 +/- 0.00025. From models and observations of irregular and blue compact galaxies it is found that Delta Y/Delta O = 3.5 +/- 0.9 and, consequently, that the primordial helium abundance by mass is given by Y-p = 0.2345 +/- 0.0026 (1 sigma). This result is compared with values derived from big bang nucleosynthesis and with other determinations of Y-p.