  This work studies the relation between gas-phase oxygen abundance and
stellar-to-gas fraction in nearby galaxies. We first derive the theoretical
prediction, and argue that this relation is fundamental, in the sense that it
must be verified regardless of the details of the gas accretion and star
formation histories. Moreover, it should hold on "local" scales, i.e. in
regions of the order of 1 kpc. These predictions are then compared with a set
of spectroscopic observations, including both integrated and resolved data.
Although the results depend somewhat on the adopted metallicity calibration,
observed galaxies are consistent with the predicted relation, imposing tight
constraints on the mass-loading factor of (enriched) galactic winds. The
proposed parametrization of the star fraction-metallicity relation is able to
describe the observed dependence of the oxygen abundance on gas mass at fixed
stellar mass. However, the "local" mass-metallicity relation also depends on
the relation between stellar and gas surface densities.
