We propose a scenario for the formation and evolution of isolated gas-rich dwarf galaxies based on the formation of stars fueled by continuous accretion of primordial gas from the surrounding intergalactic medium. Combining the relations between gas mass, nitrogen and oxygen abundances, B-band luminosity and (B-V) color, we conclude that smaller galaxies must form less stars per unit gas mass than bigger systems. Objects that show higher (N/O) than predicted by the models are good candidates for selective gas loss due to enriched galactic winds, whereas a low (N/O) ratio can be interpreted as evidence for a recent burst.
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