et al. 2002). 32% of the survey PNe have $T_{\rm B} \ga 3,500$ K and 84% have $T_{\rm B}\ga
65$ K. Thus the bulk of the observed PNe, with the exception of those with the lowest values of $T_{\rm B}$, are expected to be younger (and in many cases much younger) than $\sim 2000$ yr. Our selection criteria strongly discriminate against large, evolved, low surface brightness PNe. Some of these are known to retain significant CO envelopes, as exemplified by the Helix Nebula (Young et al. 1999), $T_{\rm B} \sim 1$ K, age $\sim$ 10$^4$ yr, but the objective of the present work is to study the earlier evolution of the envelopes.

It is known from our previous observations (Huggins et al. 1996) that CO envelopes of PNe show large differences which are likely related to the mass of the central stars, and possibly additional factors as well. The distribution of the survey PNe over different spectroscopic groups is intended to provide information to investigate this further. Of course the relation between age and nebula excitation is a complex one: young PNe can have a range of stellar temperatures because the speed of the temperature increase of the star depends 