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

16 5. Two of the young planetary nebulae in this sample, CRL 618 and IRAS 174231755, have fast as well as slow molecular winds. Several other stars in the sample also have fast molecular winds (e.g. OH231.8+4.2) but do not have central hot stars. Thus if the phenomenon of fast winds is related to planetary nebula formation at all, it begins before the central star is hot enough to ionize the nebula. 6. Continuum emission was detected for four other stars, VY CMa, IRC+10216, CIT 6 and R Leo, but is too weak to be due to an ionized HII region. In all cases, the emission is several times stronger than expected for thermal emission from the stellar photospheres. While the excess emission from IRC+101216 and VY CMa can plausibly be attributed to dust, it is more likely that the

emission from all four stars is from extended chromospheres. If so, the chromospheres extend to several (optical) stellar radii, and the stellar atmospheres are cool enough for dust to form and drive mass loss. However, the radio emission from VY CMa may be anomalously strong: we did not detect the other supergiant star in the sample, VX Sgr, whose distance and luminosity are similar to those of VY CMa.

AcknowledgementsWe thank the NRAO for granting time for this project and the sta for their expert help. Jacqueline van Gorkom provided much assistance in setting up the observations. We are very grateful to Robert Lupton for installing AIPS at Princeton and for advice on data reduction. We thank the referee for an excellent and detailed critique of the original version of this paper. We made extensive use of the SIMBAD data base, operated at CDS, Strasbourg, France and accessed via the Center for Astrophysics, Harvard University. This research was also partly supported by contract number N00014-89c-2398 at the Naval Research Laboratory.