Based in parts on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Science Institute, which is operated by the Association of the Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. The data are retrieved from the ESO/ST-ECF Science Archive Facility.

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Appendix A is only available in electronic form at http://www.edpsciences.org

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... therein)

Note that Mellema uses the ratio ${\cal R} = \dot{R}/V_{\rm post}$ while we have instead replaced $V_{\rm post}$ , the post-shock velocity (stellar frame), by the spectroscopic quantity $V_{\rm rim}$ which is not equal to $V_{\rm post}$ because of the radial velocity variation within the rim. We found for the sequence shown in Fig. 3 that always $V_{\rm rim} > V_{\rm post}$ .

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... time

Also called ballistic expansion.

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... $L_{\odot }$

With a distance of 1.4 kpc to IC 2448 the central star has only a luminosity well below 10³ $L_{\odot }$ , much too low for a post-AGB star.

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... $^\circ$ 3639(Obj)

The stellar models used in our hydrodynamical modelling are powered by hydrogen-burning and have normal, hydrogen-rich surface compositions.

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... predict

Remember that the expansion of the shell's outer edge is thermally driven, i.e. due to the heating by the ionizing photon flux from the star, and is not influenced by the powerful Wolf-Rayet wind.

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