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

11 epochs at other wavelengths, most often at 6 cm. The 6 cm ux density at epoch 1991.0 can be estimated from the 3.6 cm ux density and a spectral index of 1.8 (cf. Knapp, Sandell and Robson 1993) to be 26 3 mJy. Figure 2 shows the time dependence of the 6 cm ux density of CRL 618 since 1974, with other data from Wynn-Williams (1977), Kwok and Feldman (1981), Mross, Weinberger and Hartl (1981), Spergel et al. (1983), Kwok and Bignell (1984) and Zijlstra et al. (1989). The ux density has grown roughly linearly with time for the past 20 years; the time dependence in Figure 2 suggests that the ionization of the source began in about 1965. b. CRL 915 The\Red Rectangle" is a symmetric bipolar nebula surrounding the close binary star HD 44179 which is classi ed as B8-A0 (Cohen et al. 1975; Heintz 1990). The CO emission is extended approximately north-south (Knapp et al. 1995), as is the optical nebula (whose size is about 7000 ). CRL 915 has a remarkable spectrum, with emission features from many carbon molecules, PAHs and CH+ (Cohen et al. 1975; Schmidt et al. 1980; Waelkens et al. 1992; Balm and Jura 1992; Sloan et al. 1993). Spectroscopy of the central star by Waelkens et al. (1992) suggests a luminosity of 2300 - 7500 L . The larger value, together with the bolometric ux of 2:6 10?7 erg cm?2 s?1 found from the infrared observations listed by Gezari et al. (1987), gives a distance of 940 pc. We assume a distance of 1 kpc and a luminosity of 8000 L in round numbers. The mass loss rate is then 3 10?7 M yr?1 (Knapp et al. 1995) and the radius of the optical nebulosity is 5 1017 cm.