Evolution of the birth rate of potential candidates of CRT progenitors. The red solid line is for all the potential systems, while the green dashed and light-blue dotted lines are for those with always unstable and stable mass transfer, respectively. The blue dot-dashed line represents the systems whose mass transfer is neither always stable nor always unstable (see also Fig. 1). The vertical bar indicates the range of the birth rate of CRTs, normalized to the birth rate of SNe Ia in S0/E galaxies (Mannucci et al. 2005). The two vertical dotted lines mark the positions of delay times of 1 Gyr and 3 Gyr.