Some post-novae, e.g. DQ Her, CP Pup, T Aur, have cool envelopes, Te < 1000K. As has been shown by Ferland et al. (1984) this effect is primarily due to large overabundances of the CNO elements which at low densities can very effectively cool off the nebular gas through infra-red lines.We have constructed an extensive grid of photoionization models in order to study the exact conditions required for the electron temperature to drop to such low values. As can be seen in Figs. 1 to 4, our

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... ts can be summarized as follows.1- The abundance of oxygen which is the dominant coolant in the conditions considered has to be at least 10 times the solar value (with C and N accordingly overabundant).2- The gas density, n, has to be below 1000cm−3.3- The effective temperature of the ionizing source,Teff, has to be below 2 105K.4- The ionization parameter U = QH/ (4πr2nc) (QH: number of ionizing photons emitted per second by the central source, r: radius of the envelope) has to be between a certain range, typically 10−2< U < 10−3, whose exact limits, however, depend on other parameters such as Teffand the abundances.