Chemical evolution of irregular and blue compact galaxies

We discuss the chemical evolution of metal-poor galaxies and conclude that their oxygen deficiency is not due to the production of black holes by massive stars or a varying slope of the initial mass function, IMF, at the high-mass end. A varying IMF at the low-mass end alone or in combination with (a) an outflow of oxygen-rich material, (b) an outflow of well-mixed material, and (c) the presence of dark matter that does not participate in the chemical evolution process is needed to explain their oxygen deficiency. Outflow of material rich in O helps to account for the large {DELTA}Y/{DELTA}O values derived from these objects, but it works against explaining the (Z - C - O)/O and C/O values.