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

Search for blue compact dwarf galaxies during quiescence II: metallicities of gas and stars, ages, and star-formation rates