Predicted and observed [N/C] and [Si/C] ratios versus [C/H]. The model refer to an elliptical of 10¹²   M_⊙. In particular, the solid line refers to the fiducial case (model M12d), whereas the dotted line gives the predictions in which the metallicity dependent yield from Woosley & Weaver (1995) + Maeder (1992) are adopted (model M12MDd). Data: NLR by D’Odorico et al. (2004).