Mid infrared colors of early type galaxies

We compare the mid infrared colors of early type galaxies with isochrones that suitably account for the effects of dust in the circumstellar envelopes of AGB stars. We show that these stars affect significantly the integrated colors in the ISO bands. The excess at 10 mum observed in some early type galaxies combined with the optical spectrum, is compatible with a 100% old metal rich population. Limits on the additional young components are: age 0.2 Gyr, mass=0.5% and E(B-V)greater than or equal to0.5 or age 3 Gyr, mass 10% and E(B-V)greater than or equal to0.2.