The distances of highly evolved planetary nebulae.

The central stars of highly evolved planetary nebulae (PNe) are expected to have closely similar absolute visual magnitudesMV. This enables us to determine approximate distances to these sources where one knows their central star visual magnitudes, and levels of extinction. We find that such an analysis implies values ofDwhich are similar to those determined by Phillips; Cahn, Kaler&Stanghellin; Acker, and Daub. However, our distances are very much smaller than those of Zhang; Bensby&Lundstrom, and van de Steene&Zijlstra. The reasons for these differences are discussed, and can be traced to errors in the assumed relation between brightness temperature and radius.Finally, we determine that the binary companions of such stars can be no brighter than, implying a spectral type of K0 or later in the case of main-sequence stars. [ABSTRACT FROM AUTHOR]