H ii gas-phase metallicity and g-band magnitude of the XMP galaxies as a function of the stellar and H i gas M/L ratio. The color code parametrizes the morphology as follows: red = symmetric, black = cometary, blue = two-knot, green = multi-knot and grey = no morphological information. The symbol code is •= M_H   i/L_g and ×  = M_⋆/L_g. Top: the plot includes a range of stellar mass-to-light ratios found in spirals (magenta line), lenticulares (cyan dashed line), dwarf irregulars (orange dashed line) and ellipticals (magenta dashed line), an upper limit found in star-forming (cyan line) galaxies (Faber & Gallagher 1979) and an average value (orange line) found for BCDs (Papaderos et al. 1996a). Bottom: we plot in cyan the H i gas M/L ratio-to-luminosity correlation found for dwarf galaxies (Staveley-Smith et al. 1992).