  For the first time, we obtain chemical evolution models (CEMs) for Tucana and
Cetus, two isolated dwarf spheroidal galaxies (dSphs) of the Local Group. The
CEMs have been built from the star formation histories (SFHs) and the
metallicity histories, both obtained independently by the LCID project from
deep color-magnitude dia- grams. Based on our models, we find that the chemical
histories were complex and can be divided into different epochs and scenarios.
In particular, during 75 percent of the SFH, the galaxies behaved as closed
boxes and, during the remaining 25 percent, either received a lot of primordial
gas by accretion or they lost metals through metal-rich winds. In order to
discriminate between these two scenarios, abundances ratios in old stars are
needed. At t approximately 4.5 Gyr, the galaxies lost most of their gas due to
a short-strong, well-mixed wind. We obtain very similar CEMs for both galaxies,
although Cetus is twice as massive as Tucana. We conclude that the star
formation in both galaxies began with only 1.5 percent of the baryonic mass
fraction predicted by lambda CDM.
