[kms$^{-1}$]{}. Thus the CO and optical measurements provide similar, robust estimates of the systemic velocity, with part of the residual scatter being real, reflecting differences in the structure of the ionized and molecular gas in the PNe. In the other 22 cases where CO is detected but the optical uncertainties are large, $\ga 10$ [kms$^{-1}$]{} (10 PNe), or are non-existent (12 PNe), the CO data provide the preferred or the only available radial velocity measurements of the PNe.

Line Profiles and Widths
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A striking feature of the CO spectra shown in Fig. 1 is the variety of the line profiles. They can be broadly classified into 4 basic types: (I) parabolic, similar to the CO spectra typically seen in the precursor AGB envelopes; (II) flat-topped; (III) double-peaked; and (IV) triangular, usually asymmetrical. There are also intermediate types: e.g., with a distinct but not deep concavity, between types II and III; and with strongly asymmetrical double-peaks, between types III and IV.

These different profiles probably arise from a combination of effects. The first is the evolution of the structure of the envelope from the relatively complete, spherically symmetric distribution seen around AGB stars, to one in which the envelope has 