MMT observations of new extremely metal-poor emission-line galaxies in the sloan digital sky survey

We present 6.5 m MMT spectrophotometry of 20 H II regions in 13 extremely metal-poor emission-line galaxies selected from Data Release 5 of the Sloan Digital Sky Survey to have [O III] lambda 4959/H beta less than or similar to 1 and[N II] lambda 6583/H beta less than or similar to 0.05. The electron-temperature-sensitive emission line [O III] lambda 4363 is detected in 13H II regions, allowing a direct abundance determination. The oxygen abundance in the remaining H II regions is derived using a semiempirical method. The oxygen abundance of the galaxies in our sample ranges from 12 + log (O/H) similar to 7.1 to similar to 7.8, with 10 H II regions having an oxygen abundance lower than 7.5. The lowest oxygen abundances, 7.14 +/- 0.03 and 7.13 +/- 0.07, are found in two H II regions of the blue compact dwarf galaxy SDSS J0956+2849 (equivalent to DDO 68), making it the second most metal-deficient emission-line galaxy known, after SBS 0335-052W.