E.W. is 22 $\pm$ 2 $\rm \AA$. This value would increase if an intrinsically fainter sample was selected. The distribution peaks at E.W.=25 $\rm \AA$. Over 40 % of galaxies have their E.W. in the range 16-40 $\rm \AA$. Less than 15 % galaxies have null E.W.: these are either S0/S0a galaxies misclassified as spirals or high mass systems.

The Environmental Dependence of SFR
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Given the conclusions of the previous sections, it also follows that the comparison between the SFR properties of galaxies found in and outside clusters is meaningful only for samples with equal limiting optical luminosity. The frequency distributions for two different samples, one consisting of 40 cluster members, the other of 66 isolated galaxies, both with a limiting magnitude $\rm m_p=15.4$, are compared in Fig. 9b and c. In Fig. 9b the membership is according to the criterion of Column 10 of Table 1 (“caustics”), while in Fig. 9c the membership is according to the criterion of Column 9 (angular separation). As we can see from the latter figures, the distributions are insensitive to the adopted membership criterion. Moreover no significant difference is found between the cluster distribution and that for the isolated galaxies, as derived 