Expansion of AT Cnc’s nova shell through free expansion and Sedov-Taylor phase with supersonic model calculations: For input parameters and details, see Sects. 4 and 5 and Appendix D. We use the same initial velocities as in Fig. 1. Left: Evolution of the shell radius. Right: Evolution of the expansion velocity. Both the current radius and the current expansion velocity (horizontal lines) are reached simultaneously only for an age range of ca. 111–285 yr (dash-dotted green lines).