Models of the SN Ia rate per unit mass as a function of the U−J colour for passive galaxies, as described by the evolution past a burst of star formation. Each curve refers to different durations of the bursts, namely (0.1,0.5,0.8,1,1.5,2,3,4.5,8) Gyr (top to bottom). Different panels show the results for different DTD models as labelled, and adopt the values for k_Ia as in Fig. 12, namely . The circle shows the observational determination for the passive galaxies in our sample, and its error bar, as given in Table 4.