Adapted from Figs. 12 and 5 of Bloecker (1995). Top: Post-AGB evolutionary tracks with observational data for RRLs and non-RRL detections (filled and empty symbols, respectively – Table 1). For MWC 922 and M 2-9, the uncertainty of the luminosity due to the uncertainty of the distance is indicated by the arrows. The stellar mass of the post-AGB remnant core of each track is indicated to its right. Vertical dotted lines delimit the ionization onset region (T_eff ~ 20 000–40 000 K) where our targets lie. Bottom: Post-AGB mass-loss vs. effective temperatures adopted by post-AGB models of 0.605, 0.696, 0.836, and 0.894 M_⊙ (Bloecker 1995) and of 0.546 M_⊙ and 0.565 M_⊙ (from Schönberner 1983). As in the top panel, the symbols indicate the location of our targets using the values of Ṁ_pAGB deduced in this work (Tables 4 and 5; vertical error bars correspond to an uncertainty factor of 2). The thick dash- and dot-dashed lines indicate the mass-loss rate typically assumed by evolutionary models for our three objects with RRL detections.