and Renzini, 1983a; Matteucci and Greggio, 1986). Therefore, the behaviour of the \[$\alpha$/Fe\] ratios is mainly dependent on the relative lifetimes of type Ia and II supernovae. Another key element is nitrogen and the ratio N/O. This element, in fact, is thought to originate mainly from low and intermediate mass stars and to be a “secondary” element, in the sense that it is produced proportionally to the initial stellar metallicity. As a consequence nitrogen is restored into the interstellar medium with a large temporal delay relative to oxygen, which is produced in massive stars and is a “primary” element, namely its production is independent of the initial stellar metallicity. For this reason, the $\alpha$/Fe and N/O ratios can be used as cosmic clocks and represent a clue in understanding the nature of high redshift objects such as QSO (Hamman and Ferland, 1993; Matteucci and Padovani, 1993) and DLA systems (Matteucci, 1995).

The DLA systems are generally believed to be the progenitors of the present-day spiral galaxies (Wolfe et al. 1986). However, it has also been suggested that they could be dwarf galaxies, since they have a large amount of gas, low metallicities and low dust-to-gas ratios (Pettini et al. 1990;Meyer 