[en] Emission from the J=1→0 transition of ¹²C¹⁶ O has been detected at 2.6 mm from the peculiar infrared objects CRL 2688 (the Egg Nebula) and CRL 618. The observed parabolic line-shape can be interpreted as optically thick emission from a uniformly expanding molecular envelope with a size smaller than the telescope beam. The line width indicates an expansion velocity on the order of 20 km s^-1. Other molecular lines similar to those observed in the envelope of the carbon star IRC+10216 are also observed in CRL 2688, even though the central star in CRL 2688 has a relatively early spectral type (F5 Ia). In CRL 618, the existence of an expanding molecular envelope around a central with spectral type BO implies that the central star has evolved within a very short time (<10⁴ years) from a cool, perhaps carbon-rich, star. The presence of mass loss and the rapid evolution of the central stars suggest that CRL 2688 and CRL 618 may be photo--planetary nebulae. It is noted that IRC+10216, CRL 2688, CRL 618, and the planetary nebular NGC 7027 may represent different stages of an evolutionary sequence. Other peculiar objects such as M1-92 (Minkowski's Foot-print), HD 44179 (the Red Rectangle), OH 0739--14, and HD 200775 have also been searched for circumstellar CO emission, but no emission greater than 0.2 K (5 sigma) was detected