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

10 than any yet observed. Figure 1 shows that the stars detected at 3.6 cm fall into two groups. The four stars in the rst group have values of R which are higher than the minimum expected for compact HII regions. The compact planetary nebulae CRL 618 and IRAS 21282+5050 lie in this group; the other two members, CRL 915 and IRAS 17423-1755, are therefore also likely to be planetary nebulae. The values of R for the remaining 17 stars in the sample, both those detected at 3.6 cm and those with upper limits, fall below the HII region value, showing that there are no more compact planetary nebulae among the 21 observed stars. The 3.6 cm ux densities of the other four detected stars (IRC+10216, R Leo, CIT6 and VY CMa) are too low to be due to HII regions but, as Figure 1 shows, too high to be photospheric. These stars are discussed in Appendix A.

4. PROPERTIES OF COMPACT PLANETARY NEBULAEa. CRL 618 This planetary nebula has probably formed very recently; Kwok and Feldman (1981) show that the ux density at radio frequencies, where the nebula is optically thick, is growing with time, demonstrating the growth of the central HII region. At an assumed distance of 1100 pc, the 3.6 cm ux density of 67 mJy implies an optically thick spherical HII region (at temperature 104 K) of radius 3:4 1015 cm. This HII region has an area about twice that me

asured by Kwok and Bignell (1984). The 3.6 cm ux density given in Table 2 is, to the best of our knowledge, the rst published at this wavelength. CRL 618 has been measured at several previous