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Published online by Cambridge University Press: 12 April 2016
White dwarfs (WD) are the final configurations of all stars up to initial masses between 5 and 9 M⊙. Two feeder channels for the creation of single WDs can be distinguished: Either evolution through the asymptotic giant branch (AGB) and the following planetary-nebula (PN) phase, or evolution from the horizontal branch through the hot subdwarf region. Prelimary estimates by Drilling and Schönberner (1985) and Heber (1986) indicate that the creation of WDs via the horizontal–branch channel is rather insignificant (few percent of the total WD birthrate) and can be neglected. Thus the evolution through the AGB determines the internal structure of single WDs, and the study of the PN stage serves to elucidate the inital conditions for the white-dwarf evolution.