contributing to the galactic enrichment, and ${M_{B_m}}=3.0M_{\odot}$, which represents the minimum mass for which a binary system (with the minimum mass ratio $\mu_{min}$ defined as in Greggio and Renzini 1983b) produces a type Ia SN,

\(b) binary systems producing type Ia SNe, within the range of masses ${M_{B_m}}$=${3 M_{\odot}}$ and ${M_{B_M}}$=${16 M_{\odot}}$ \[the assumed progenitors of type Ia SNe are binary systems of C-O white dwarfs(Whelan and Iben, 1973); the parameter $A$, defined in Greggio and Renzini (1983b), represents the fraction of binary systems in the IMF which can give rise to type Ia SNe;

\(c) single stars in the mass range ${M_{B_m}}$ - ${M_{B_M}}$ which end their lives either like white dwarfs or type II SNe,

\(d) single stars in the mass range ${M_{B_M}}$ and ${M_U}$, where ${M_U}$ is the maximum stellar mass contributing to the galactic enrichment (100 ${M_{\odot}}$ in our models). These stars can either end their lives as type II or type Ib SNe.

The initial mass function (IMF) by mass, $\phi(m)$, is expressed as a power law with an exponent x=1.35 over the mass range $0.1-100M_{\odot}$.

The chemical evolution equations include also an accretion term: $$\dot G_{iinf}(t)\,=\,{
{(X_{i})_{inf}e^{-t/\tau}}
\over
{\tau(1-e^{-t_{G}/\tau})}
}$$ where (X$_{i})_{inf}$ is the 