CO mapping observations are reported for three evolved planetary nebulae: NGC 7293, NGC 6720, and NGC 6072. Each has a substantial molecular envelope whose structure reflects the morphology of the ionized nebula. In the nearest, NGC 7293, which can be studied in most detail, the molecular gas is closely associated with the optical double-ring system, the kinematics show distinct though incomplete shell structures, and the gas is fragmented on the smallest size scales observed. The mass of molecular gas in these objects is comparable to or exceeds the mass of ionized gas.
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Healy et al. (1990) studied this question.