Published online by Cambridge University Press: 12 April 2016
With the advent of the IRAS satellite a completely new class of stellar objects became evident; viz., objects which are obviously in a rapid transition from the very tip of the asymptotic giant branch (AGB) into the planetary-nebulae region. These so-called pre-planetary nebulae (PPN) are characterized by stellar spectra indicative of supergiants mainly of F and G spectral types in conjunction with a large infrared excess due to a cool circumstellar dust shell. At the same time theoretical calculations through the AGB, with the inclusion of mass loss, and also through the following evolutionary stages down to the white-dwarf sequence, became available (Schonberner 1979, 1983; Wood and Faulkner 1986). These calculations predict evolutionary lifetimes of several 1000 years in the transition region between the tip of the AGB and the planetary-nebulae region. Thus a direct comparison between theory and observation now appears possible in the very early phase of this post-AGB evolution.