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

4 detect emission from planetary nebulae of very small size (cf. Wynn-Williams 1977; Kwok and Feldman 1981; Kwok and Bignell 1984; Bowers and Knapp 1989). Radio frequency observations should, in principle, easily be able to distinguish between emission from a compact planetary nebula and from a stellar photosphere by the relative strength of the radio continuum emission. Optically thick bremmstrahlung emission from a sphere of radius R? and temperature T? gives a ux density at frequency of 2 R2 (1) S= 2kT? c2 D2? where D is the distance to the star. AGB stars have radii of several A.U. and temperatures of 2000 - 3000 K. The initial formation of a planetary nebula, on the other hand, produces a region of ionized gas of temperature 104 K and radius 1015 cm (cf. Kwok and Feldman 1981) whose ux density is 50 - 100 times stronger than would be expected for photospheric emission from a red giant star. The sample selection is described in x2, and the observations, data reduction and results in x3. Continuum emission which is most likely due to compact planetary nebulae was detected in four objects: these are individually discussed in x4, as are the implications of these results for the initial stages of planetary nebula formation. The conclusions are given in x5. Continuum emission was detected from ve additional stars. The weak emission tentatively detected from CRL 2688 appears to be extended and may be due to circumstellar dust - this result is described elsewhere (Knapp et al. 1994, hereafter Paper 1). The emission from the other four stars is too weak to be due to HII regions but is too strong to be photospheric, and indicates the possible presence of extended chromospheric emission. These results and their relation to stellar mass