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A standard planetary nebula stays more than 10 000 years in the state of aphotoionized nebula. As long as the timescales of the most important ionizingprocesses are much smaller, the ionization state can be characterized by astatic photoionization model and simulated with codes like CLOUDY (Ferland etal. 1998). When the star exhibits a late Helium flash, however, its ionizingflux stops within a very short period. The star then re-appears from itsopaqueshell after a few years (or centuries) as a cold giant star without any hardionizing photons. Describing the physics of such behavior requires a fullytime-dependent radiative transfer model. Pollacco (1999), Kerber et al. (1999)and Lechner & Kimeswenger (2004) used data of the old nebulae around V605 Aqland V4334 Sgr to derive a model of the pre-outburst state of the CSPN in astatic model. Their argument was the long recombination time scale for suchthin media. With regard to these models Schoenberner (2008) critically raisedthe question whether a significant change in the ionization state (and thus thespectrum) has to be expected after a time of up to 80 years, and whether staticmodels are applicable at all.

Investigation of the recombination of the retarded shell of "born-again" CSPNe by time-dependent radiative transfer models