in the nebula. This is likely to be mainly a direct effect in which \[\] is enhanced in dense interface gas, or shocked gas often associated with bipolar flows and dense tori. It may also be partly indirect, in that higher mass progenitors, which often have an enhanced N abundance, preferentially have more massive molecular envelopes. These will be discussed in more detail in a separate paper.

Radial Velocities
-----------------

The CO spectra of the 40 detected PNe provide a useful and homogeneous set of PN radial velocities ($V_{\rm o}$ in Table 1). In the cases where the CO spectrum is relatively clear of IS line contamination and the S/N is good, the uncertainty in the measured velocities is small $\sim 1$–2 [kms$^{-1}$]{}, and is limited by the intrinsic asymmetry in the line profiles. Even in cases with severe IS contamination or low S/N, the uncertainty is almost always $\la 10$ [kms$^{-1}$]{}.

High or medium quality (uncertainties $\la 10$ [kms$^{-1}$]{}) optical measurements of the radial velocities of the detected PNe are available in only 18 cases (see Section 4). For this set, the mean value of $V_{\rm o} - V_{\rm opt} = 5$ [kms$^{-1}$]{} with an rms scatter of 12 