strongly on the stellar mass (e.g., Gesicki & Zijlstra 2000). The nebula excitation is therefore not a direct measure of the nebula age, but alone, or in combination with the brightness temperature may cast light on, or serve as a predictor of the CO emission.

Based on the \[O[iii]{}\] $\lambda$5007/H$\beta$ line ratio the first group (40 objects) selects low excitation PNe (excitation classes $\la 2$ on the scale of Webster 1975) with central star temperatures $\la 45,000$ K (e.g. Kaler & Jacoby 1991). The second group (31 objects) selects higher excitation PNe with strongly enhanced \[\] emission and is likely to include the higher mass progenitor stars which pass rapidly through the low excitation phase; the third group (28 objects) acts as a control. A small, additional fourth group (11 objects) of miscellaneous PNe with incomplete spectroscopy, or with larger angular diameters and typically in the lowest quartile of $T_{\rm B}$, was also observed as time in the program allowed.

Observations
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The survey observations were made in the CO $J=2-1$ line at 230.538 GHz using the IRAM 30 m telescope at Pico Veleta, Spain, in March 2002 and May 2003.

The receiver system used was the multibeam Heterodyne Receiver 