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We present 6.5-meter MMT spectrophotometry of 20 H II regions in 13 extremelymetal-poor emission-line galaxies selected from the Data Release 5 of the SloanDigital Sky Survey to have [O III] 4959/Hbeta < 1 and [N II] 6583/Hbeta < 0.05.The electron temperature-sensitive emission line [O III] 4363 is detected in 13H II regions allowing a direct abundance determination. The oxygen abundance inthe remaining H II regions is derived using a semi-empirical method. The oxygenabundance of the galaxies in our sample ranges from 12+logO/H ~ 7.1 to ~ 7.8,with 10 H II regions having an oxygen abundance lower than 7.5. The lowestoxygen abundances, 12+logO/H = 7.14+/-0.03 and 7.13+/-0.07, are found in two HII regions of the blue compact dwarf galaxy SDSSJ0956+2849=DDO 68, making itthe second most-metal deficient emission-line galaxy known, after SBS0335-052W.

MMT observations of new extremely metal-poor emission-line galaxies in the Sloan Digital Sky Survey