TheKLuminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31

We determine a K-band luminosity-metallicity (K-Z) relation for dwarfirregular galaxies, over a large range of magnitudes, -20.5 < M_K < -13.5,using a combination of K photometry from either the 2-micron all sky survey(2MASS) or the recent study of Vadivescu er al. (2005), and metallicitiesderived mainly with the T_e method, from several different studies. We then usethis newly-derived relation, together with published K_s photometry and our newspectra of objects in the field of HCG 31 to discuss the nature of the possibletidal dwarf galaxies of this group. We catalogue a new member of HCG 31, namely"R", situated ~40 kpc north of the group center, composed by a ring of H alphaknots which coincides with a peak in HI. This object is a deviant point in theK-Z relation (it has too high metallicity for its luminosity) and its projecteddistance to the parent galaxy and large gas reservoir makes it one of the mostpromising tidal dwarf galaxy candidates of HCG 31, together with object F. Thesubsystems A1, E, F, H and R all have metallicities similar to that of thegalaxies A+C and B, result that is expected in a scenario where those wereformed from material expelled from the central galaxies of HCG 31. Whileobjects A1, E and H will most probably fall back onto their progenitors, F andR may survive as tidal dwarf galaxies. We find that two galaxies of HCG 31, Gand Q, have A+em spectral signatures, and are probably evolving toward apost-starburst phase.