Time-dependent effects in the nebular shell of FG sagittae

Theoretical models have been constructed of planetary nebulae illuminated by central stars which are evolving on a time scale comparable with the cooling time of the nebular gas. The emission line spectra of the models have been compared with the spectrum of the planetary nebula surrounding FG Sge. The observed spectrum can be reproduced by that of a nebular shell of density n/sub H/=160 cm-3, illuminated by a central star which has decreased linearly in temperature at constant luminosity from an initial value of 55,000-75,000 K. The nebula appears to have a N/O ratio which is anomalously high even for planetary nebulae