Effective yield and the H ii-to-H i metallicity ratio of the XMP galaxies as a function of the gas mass fraction. The color code parametrizes the morphology as follows: red = symmetric, black = cometary, blue = two-knot, green = multi-knot and grey = no morphological information. Top: the symbol code is • = XMPs with MPA-JHU-determined stellar masses, ° = XMPs with color-determined stellar masses, □ = spiral and ▴ = irregular galaxies. The spiral and irregular galaxy data were obtained from Pilyugin et al. (2004). The magenta line represents the theoretical yield limit for a closed-box model. Bottom: the symbol code is • = XMPs with MPA-JHU-determined stellar masses and ° = XMPs with color-determined stellar masses.