The evolution of stars between the 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 newly formed compa

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5. CONCLUSIONSSensitive observations were made of radio continuum emission from a sample of 21 red giant and supergiant stars with high mass loss rates and extended cool circumstellar envelopes in a search for compact planetary nebulae. The sample of stars was selected to emphasize characteristics which may denote objects in transition between the AGB and planetary nebula stages, such as stars with relatively early spectral types, stars with fast as well as slow molecular out ows, and stars with cool circumstellar envelopes. 1. Emission from compact HII regions was detected from four stars, CRL 618, IRAS 21282+5050, IRAS 17423-1755 and CRL 915. The last two are new detections. The observations of the remaining stars are sensitive enough to show that they do not contain compact ionized regions. 2. The ux density of CRL 618 continues to increase, showing the growth of the central HII region. These observations suggest that ionization of the nebula began in about 1965. 3. The presence of an ionized region in CRL 915, the`Red Rectangle', shows that the exciting star is of type B3 or earlier. This is much hotter than the spectral type of HD 44179, the apparent central star of CRL 915. Either this star is misclassi ed, it has evolved rapidly, or it is the binary companion to the hidden exciting star of the nebula. 4. The rapid evolution of the central stars of planetary nebulae are shown by the dynamical times of the four nebulae; the older nebulae have hotter central stars.