Extremely metal‐deficient (XMD) galaxies, by definition, have oxygen abundances ≤1/10 solar, and form a very small fraction of the local gas‐rich, star‐forming dwarf galaxy population. We examine their positions in the luminosity–metallicity (L–Z) and mass–metallicity (M–Z) planes, with respect to the L–Z and M–Z relations of other gas‐rich, star‐forming dwarf galaxies, viz., blue compact galaxies (BCGs) and dwarf irregular (dI) galaxies. We find that while the metallicities of some low‐luminosity XMD galaxies are consistent with those expected from the L–Z relation, other XMD galaxies are deviant, and more so as the luminosity and/or metal‐deficiency increases. We determine the 95 per cent confidence interval around the L–Z relation for BCGs, and find that its lower boundary is given by 12 + log (O/H) =− 0.177 MB+ 4.87. We suggest that a galaxy should be regarded as XMD, in a statistically significant manner, only if it lies below this boundary in the L–Z plane. Of our sample of XMD galaxies, we find that more than half are XMD by this criterion, and in fact, nine of the galaxies lie below the 99.5 per cent confidence interval about the L–Z relation. We also determine the gas mass fractions and chemical yields of galaxies in all three samples. We find that the effective chemical yield increases with increasing baryonic mass, consistent with what is expected if outflows of metal‐enriched gas are important in determining the effective yield. XMD galaxies have lower effective yield than BCG/dI galaxies of similar baryonic mass. This suggests that some process, peculiar to XMD galaxies, has resulted in their low measured metallicities. Motivated by the fact that interactions are common in XMD galaxies, we suggest that improved (tidally driven) mixing of the interstellar media (ISM) in XMD galaxies leads to a lowering of both, the measured metallicity and the calculated effective yield. In isolated dwarf galaxies, the outer parts of the stellar envelope probably do not participate in the star formation, but are still generally included in the calculation of effective yield. This results in an overestimate of the effective yield. We suggest that XMD galaxies are deviant from the L–Z relation because of a combination of being gas rich (i.e. having processed less gas into stars) and having more uniform mixing of metals in their ISM.

We present Giant Metrewave Radio Telescope, H i 21 cm observations of SBS 0335–052E and SBS 0335–052W, a close pair of dwarf galaxies, which are further unusual in being the most metal‐poor star‐forming galaxies known. We present images at several angular resolutions, ranging from ∼40 to 4 arcsec. These images show that SBS 0335–052 is a strongly interacting system, with a faint diffuse H i bridge seen at low resolution, and elongated tails seen at the higher resolutions. The overall morphology suggests that the pair represents a major (as both galaxies have similar H i masses) merger of extremely gas‐rich galaxies, which is currently past the first close encounter. The low‐resolution velocity field is dominated by the velocity difference between the two galaxies and the velocity gradient along the tidal features. However, for SBS 0335–052W at least, at high angular resolution, one sees a central velocity field that could be associated with the spin of the original undisturbed disc. The two galaxies have very similar H i masses, but very different optical properties and current star formation rates. A possible reason for this is the differing amounts of tidally induced star formation, because of the different spin orientations of these interacting galaxies. The highest angular resolution H i images show that the ionized superbubble, identified by Thuan, Izotov & Lipovetsky, in the Hubble Space Telescope images of SBS 0335–052E, is extended along one of the diffuse tidal features, and that there is a high‐density H i clump at the other end of the superbubble. The star formation in SBS 0335–052E occurs mainly in a group of superstar clusters (SSCs) with a clear age gradient; the age decreases as one approaches the dense H i clump. We suggest that this propagating star formation is driven by the superbubble expanding into a medium with a tidally produced density gradient. The high pressures associated with the compressed material would also naturally explain why current star formation is mainly concentrated in SSCs.

We present Giant Metrewave Radio Telescope H i observations of two extremely metal‐deficient (XMD) galaxies, UM 133 and SDSS J011914.27−093546.4, with no known nearby companions. Earlier H i observations of XMD galaxies have shown that disturbed H i morphologies and kinematics, generally related to tidal interaction with a companion, are common in them. However, some of these galaxies were already known to be in pairs before the H i observations. The two galaxies studied here were specifically selected because they do not have any known nearby companions. None the less, we find that both the galaxies have highly disturbed H i morphologies and velocity fields. These could have arisen from interactions with more distant companions than considered in our isolation criteria or merger with extremely gas‐rich dwarf companions. It is also possible that distorted H i distributions of these XMD galaxies are the result of cold gas accretion from the intergalactic medium. Our observations provide further support to the idea that inflow of metal‐poor gas from the outskirts of a galaxy to the central star‐forming regions (in the case of interaction), or cold gas accretion is the probable cause for the observed low emission‐line metallicities of XMD galaxies.