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(abridged) We present long-slit spectroscopy with the 3.6m ESO telescope ofeight HII regions in seven star-forming (SF) dwarf galaxies, discovered in SDSSDR4 and 6dFGRS. In addition, we use SDSS imaging data to study the photometricstructure of the sample galaxies. From the 3.6m telescope spectra, we determinethe oxygen abundance of these systems to be 7.3<12+log(O/H)<7.6, placing themamong the most metal-poor SF galaxies ever discovered. Our photometric analysisreveals a moderately blue, stellar host galaxy in all sample galaxies. Thedetection of a stellar host in all galaxies studied here and all previouslystudied extremely metal-deficient SF galaxies implies that they are unlikely tobe forming their first generation of stars. With regard to the structuralproperties of their host galaxy, we demonstrate that these systems areindistinguishable from blue compact dwarf (BCD) galaxies. However, in contrastto the majority (>90%) of BCDs that are characterised by red elliptical hostgalaxies, extremely metal-poor SF dwarfs (hereafter XBCDs) reveal moderatelyblue and irregular hosts. This is consistent with a young evolutionary statusand in the framework of standard star formation histories implies that severalXBCDs formed most of their stellar mass in the past 2 Gyr. A large fraction ofXBCDs reveal a cometary morphology due to the presence of intense SF activityat one edge of an elongated host galaxy with a gradually decreasing surfacebrightness towards its antipodal end.

Extremely metal-poor star-forming galaxies. New detections and general morphological and photometric properties