  In this letter we construct a large sample of early-type galaxies with
measured gas-phase metallicities from the Sloan Digital Sky Survey and Galaxy
Zoo in order to investigate the origin of the gas that fuels their residual
star formation. We use this sample to show that star forming elliptical
galaxies have a substantially different gas-phase metallicity distribution from
spiral galaxies, with ~7.4% having a very low gas-phase metallicity for their
mass. These systems typically have fewer metals in the gas phase than they do
in their stellar photospheres, which strongly suggests that the material
fuelling their recent star formation was accreted from an external source. We
use a chemical evolution model to show that the enrichment timescale for
low-metallicity gas is very short, and thus that cosmological accretion and
minor mergers are likely to supply the gas in >37% of star-forming ETGs, in
good agreement with estimates derived from other independent techniques.
