cooling efficiency of the various mechanisms, to properly evaluate the total gravitational potential well and to describe the thermal history of the gas. In this scheme it comes out that the galactic wind occurs typically for ages $t_{GW} < 1$ Gyr, later in a massive early-type galaxy and much earlier in galaxies of lower mass [@Arimoto87; @Arimoto89; @Bressan94; @Gibson97b; @Tantalo96; @Tantalo98; @Chiosi98]. The maximum duration of star forming activity follows therefore in these models the trend $\Delta t_{SF} \propto M_{G}$. This trend of the SFH is, however, contrary to what required by the observes trend of the $\alpha$-enhancement for early type galaxies, which implies that the maximum duration of the star forming activity should decrease when the galaxy mass increases $\left( \Delta t_{SF}
\propto M_{G}^{-1}\right)$ [see @Bressan96; @Kuntschner00; @Trager00a; @Trager00b; @Tantalo04; @Thomas05 for more details on the enhancement in $\alpha$-elements and the SFH of early-type galaxies].

Second, after the galactic wind phase, star formation does no longer occur and the evolution is merely passive. However, AGB and RGB stars continue to loose gas in amounts that are comparable to those before the galactic wind [@Chiosi00]. What is the fate of this gas? One may imagine that the large amount of 