2 The sample

The initial criteria we have applied for the selection of a representative sample of AGB and post-AGB stars were: line strength, IR colors and chemical composition. All the stars in the sample were already known from previous observations to be relatively strong CO emitters with intensities, in units of main beam temperatures, larger than 0.5K and 1.0K in the ($J=1\rightarrow 0$) and ($J=2\rightarrow 1$) transitions, respectively. The circumstellar envelopes were further selected by their IR colors, to sample as evenly as possible the IR color diagram (see Fig. 1 and Table 3), and by chemical composition. Finally, to take advantage of the sensitivity and resolution of the IRAM interferometer we have restricted our sample to high declination stars which would be either unresolved, or barely resolved, at the 30 m telescope.

Our initial sample contained 53 AGB and post-AGB stars: 24 O-type stars, 25 C-type stars, 2 S-type stars and 2 of uncertain composition, according to the compilation by Loup et al. (1993). The selected stars are listed in Tables 2 and 4.