type bins, the segregation according to $\rm C_{31}$ is apparent, and enhanced in panels d and e.

It is evident that the quantities $\rm C_{31}$, $\rm H\alpha$, $\rm L_H$ and morphological type are not independent of one another, as illustrated in Fig. 7. However, while both $\rm H\alpha$ (Fig. 7a, extracted from Fig. 5b) and $\rm C_{31}$ (Fig. 7b) are strongly related with $\rm L_H$, the correlation of $\rm H\alpha$ with $\rm C_{31}$ (Fig. 7c) is poorer: most disk galaxies ($\rm C_{31}<$3) have high SFR, but a significant fraction of them have $\rm Log H\alpha<$1. Any value of $\rm H\alpha$ E.W. is found among bulge-dominated objects.

In conclusion, we claim that the dependence of $\rm H\alpha$ E.W. on mass (H luminosity) is not entirely induced by the dependence of the bulge-to-disk ratio on luminosity. The primary dependences are those of $\rm C_{31}$ and of $\rm H\alpha$ E.W. on mass, and the one of $\rm H\alpha$ E.W. on $\rm C_{31}$ follows as a consequence.

The secondary, marginal correlation between $\rm H\alpha$ and $\rm C_{31}$ implies, however, that in bulge-dominated systems the $\rm H\alpha$ E.W. might not provide a reliable estimate of the disk SFR, as argued by Devereux & Young (1991). 