a) Absolute SDSS g magnitude-oxygen abundance relation for the galaxies with oxygen abundances 7.35 ≤ 12 + log O/H ≤ 7.65 derived by the direct method (magenta filled triangles) and for galaxies with oxygen abundances derived by the semi-empirical method of Izotov & Thuan (2007; magenta open triangles). Three XMD candidates with 12 + logO/H ≤ 7.35 (this paper), denoted by large red filled circles and identified by the direct metallicity determination, are shown together with large red open circles representing sources with semi-empirical determinations. The total number of the LMG sample shown by magenta and red symbols is 287. We also show the XMD emission-line galaxies and candidates with 12 + log O/H ≤ 7.35 from Izotov & Thuan (2007), Izotov et al. (2012) and Guseva et al. (2015) by small blue circles. Well-known metal-deficient BCDs I Zw 18, SBS 0335–052W, and SBS 0335–052E are shown by filled blue triangles and are labelled in b). The extremely metal-poor dwarf galaxies Leo P (Skillman et al. 2013) and AGC 198691 (Hirschauer et al. 2016) are plotted respectively by a large open black square and large open black diamonds corresponding to three different distances. For comparison, the standard L–Z relation for emission-line galaxies (Izotov et al. 2006a; Izotov & Thuan 2007) is shown by black dots with the linear regression to these data (solid line). b) Extinction- and aperture-corrected Hβ luminosity-oxygen abundance relation. The samples and symbols are the same as in a). The black dots represent the SDSS galaxies from Izotov et al. (2006a), and all galaxies from Izotov & Thuan (2007) and Izotov et al. (2012). The black solid line denotes the linear regression to these data, excluding I Zw 18 and SBS 0335–052W. The filled green squares represent the high-redshift star-forming galaxies by Cullen et al. (2014).