The evolution of stars between the asymptotic giant branch (AGB) and planetary nebula phases was investigated by sensitive radio continuum observations of a sample of 21 evolved stars with high mass-loss rates and extended circumstellar envelopes, in a search for a newly formed compact planetary nebulae. Four were found: two (CRL 618 and IRAS 21282+5050) are well-known young planetary nebulae while the other two (IRAS 17423-1755 and CRL 915, the "Red Rectangle") are new detections. These recent observations show that the radio frequency flux of CRL 618 continues to increase. The strength of the radio continuum emission from CRL 915 gives a type of B3 or earlier for the exciting star. Radio continuum emission was detected from four more stars in the sample at an intensity too weak to be due to compact planetary nebulae but too strong to be photospheric These stars may have extended partially ionized chromospheres.
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Knapp et al. (1995) studied this question.