Evolution of the radius and expansion velocity of a SNR (green shaded areas) as a function of time for the pressure-drive snowplough phase using the analytical expressions from Cioffi et al. (1988) for an initial explosion energy of 0.5 × 10⁵¹. The evolution is calculated for the typical range of values found for n_ISM in Sect. 3.5 (Table 5). The dashed line indicates the radii of the SNRs measured from the HST images (Fig. 9) and the blue and yellow shaded areas represent the range of shock velocities determined in Sect. 3.4 (Table 5) for SNR 1 and 2, respectively.