abundance of the element [*i*]{} in the infalling gas, assumed to be primordial, $\tau$ is the time scale of mass accretion and $t_{G}$ is the galactic lifetime. This accretion term may simulate the formation of dwarfs as the result of mergers of smaller subunits. The parameter $\tau$ has been assumed to be the same for all blue compact galaxies and short enough to avoid unlikely high infall rates at the present time ($\tau=0.5\cdot 10^{9}$ years). It is worth noting that a shorter timescale would not produce a noticeable effect on the results.

Nucleosynthesis prescription
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The term ${{Q_m}_i(t-{\tau_m})}$ in equation (1), the so called [*production matrix*]{} (Talbot and Arnett, 1973), represents the fraction of mass ejected by a star of mass $m$ in the form of the element $i$. The quantity $\tau_{m}$ represents the lifetime of a star of mass m and $\tau_{m2}$ refers to the mass of the secondary component and $\tau_{m1}$ to the mass of the primary component of a binary system giving rise to a type Ia SN (see Matteucci and Greggio, 1986 for details). For the nucleosynthesis prescriptions we have assumed the following:

a\) For low and intermediate mass stars (0.8 $\leq$ M/M$_{\odot} \leq$ M$_{up}$) we 