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Using the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph(MODS), we have obtained optical spectroscopy of one of the most metal-poordwarf star-forming galaxies (SFG) in the local Universe, J2229+2725. Thisgalaxy with a redshift z=0.0762 was selected from the Data Release 16 (DR16) ofthe Sloan Digital Sky Survey (SDSS). Its properties derived from the LBTobservations are most extreme among SFGs in several ways. Its oxygen abundance12+logO/H=7.085+/-0.031 is among the lowest ever observed for a SFG. With itsvery low metallicity, an absolute magnitude Mg=-16.39 mag, a low stellar massMstar=9.1x10^6 Msun and a very low mass-to-light ratio Mstar/Lg~0.0166 (insolar units), J2229+2725 deviates strongly from the luminosity-metallicityrelation defined by the bulk of the SFGs in the SDSS. J2229$+$2725 has a veryhigh specific star-formation rate sSFR~75 Gyr^-1, indicating very activeongoing star formation. Three other features of J2229+2725 are most striking,being the most extreme among lowest-metallicity SFGs: 1) a ratioO32=I([OIII]5007)/I([OII]3727)~53, 2) an equivalent width of the Hbeta emissionline EW(Hbeta) of 577A, and 3) an electron number density of ~1000 cm^-3. Theseproperties imply that the starburst in J2229+2725 is very young. Using theextremely high O32 in J2229+2725, we have improved the strong-line calibrationfor the determination of oxygen abundances in the most metal-deficientgalaxies, in the range 12 + logO/H<7.3.

J2229+2725: an extremely low-metallicity dwarf compact star-forming galaxy with an exceptionally high [OIII]5007/[OII]3727 flux ratio of 53