The most metal-poor BCGs: IZw18 and SBS0335-052 (and its companion), are probably not young galaxies. Nevertheless they are important laboratories for studying star formation at low abundances and the evolution of dwarf galaxies. The metallicities of their old stellar populations cannot be assessed with any accuracy. There is nothing that requires these galaxies to be more metal-poor than the most extreme dSph galaxies which have [Fe/H] -2, see Table 2 (their oxygen abundances are still not known).
In addition to these galaxies, there are more than a dozen other BCGs with abundance 12 + log(O/H) 7.6 or lower, and more objects are about to be discovered (cf. Sect. 6.1.5). The Local Group, and its immediate surroundings, contains another half dozen dI with very low metallicity. Moreover there are many dE/dSph galaxies in the neighbourhood with [Fe/H] -2. In addition, LSBGs have proven a good hunting ground for very metal poor galaxies. In Table 3 below we present all known galaxies with 12 + log(O/H) 7.6 or lower. When several measurements (representing different H II regions) for the same galaxy were available we took weighted linear averages; and the same was done if there were differing O/H determinations of similar quality in the litterature. These galaxies, taken together form an important, yet hardly homogeneous, set of data for investigating dwarf galaxy evolution. Another interesting class, to which IZw18 and SBS0335-052 and its companion belong, is galaxies with low abundances with respect to their integrated luminosities, see Sect. 7.1.