period (e.g., Josselin & Bachiller 2000). One focus of the earlier work was large, highly evolved PNe with low surface brightness. The new sample has a higher proportion (85%) of PNe with moderate to high surface brightness ($T_{\rm B} \ga 65$ K using the benchmark of Section 2.1), compared to 64% of the earlier compilation, and roughly doubles the number of relatively young PNe with known CO envelopes. The 70 sensitive upper limits reported here also improve and extend the earlier data on PNe where the molecular component has been destroyed.

The overall detection rate of the new sample is 36%, and the rate shows no significant variation with $T_{\rm B}$. On the other hand, the detection rates for the spectroscopic groups 1–4 are highly non-uniform at 15%, 71%, 36%, and 18%, respectively; this variation is significantly different from that expected on the hypothesis of a uniform distribution at the 0.1% level. These results are consistent with the view that large intrinsic variations of the CO properties of the PNe (Huggins et al. 1996) dominate over evolutionary effects for the ensemble as a whole. They also indicate that the presence of a CO envelope is correlated with higher \[\] emission 