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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Appendix A: PHOTOSPHERIC AND CHROMOSPHERIC EMISSIONThe 3.6 cm observations described in this paper detect emission from ve stars at a level which is too weak to be due to compact planetary nebulae (cf. Figure 1). The data for CRL 2688 suggest that there is weak emission at 3.6 cm from circumstellar dust, and these data are described in Paper I. Figure 1 shows that the emission from IRC+10216, R Leo, VY CMa and CIT6 appears to be in excess of the expected photospheric value. VY CMa is a supergiant OH/IR star at a distance of 1500 pc, with a luminosity of 5 105 L and a surface temperature of 2700 K. These quantities give a predicted 3.6 cm photospheric ux density (including the expected contribution from dust) of 0.046 mJy, 20% of the observed value (Table 2). A model for CIT6 which includes the expected radio frequency emission from circumstellar dust (D= 230 pc, L= _ 3200 L, T= 2500 K, M= 2 10?8 M ) gives a 3.6 cm ux density of 0.04 mJy, 25% of the observed value. The 3.6 cm ux density of R Leo (0.26 mJy measured with the VLA`C' array) is similar to the value of 0.2 mJy measured by Reid and Menten (1992) with the`D' array. The spectral index between 3.6 cm and 1.3 mm (using the measurement by Altenho et al. 1994) has the black body value of 2.0 0.06. The 3.6 cm ux density calculated from the angular diameters and e ective temperatures measured by DiGiacomo et al. (1991) and Tuthill et al. (1994) is 0.13 mJy, about 50% of the observed value. The predicted 3.6 cm ux from IRC+10216 (L= 2:6 104 L, T= 2500 K, D= 200 pc, is 0.16 mJy, 20% of the observed value; the possible contribution of dust is discussed in Paper 1). These observations show that the emission at 3.6 cm from cool red giant stars,