We examine the metallicity and age of a large set of
SDSS/DR6 galaxies that may be Blue Compact Dwarf (BCD) galaxies during
quiescence (QBCDs).The individual spectra are first classified and then
averaged to reduce noise. The metallicity inferred from emission lines
(tracing ionized gas) exceeds by ~0.35 dex the metallicity inferred from
absorption lines (tracing stars). Such a small difference is significant
according to our error budget estimate. The same procedure was applied
to a reference sample of BCDs, and in this case the two metallicities
agree, being also consistent with the stellar metallicity in QBCDs.
Chemical evolution models indicate that the gas metallicity of QBCDs
is too high to be representative of the galaxy as a whole, but it can
represent a small fraction of the galactic gas, self enriched by previous
starbursts. The luminosity weighted stellar age of QBCDs spans the whole
range between 1 and 10 Gyr, whereas it is always smaller than 1 Gyr for
BCDs. Our stellar ages and metallicities rely on a single stellar
population spectrum fitting procedure, which we have specifically
developed for this work using the stellar library MILES.