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Published online by Cambridge University Press: 14 August 2015
A most puzzling feature of the evolution of the central stars of planetary nebulae is their rapid contraction (O'Dell, 1963; Seaton, 1966); assuming homologous contraction, an enormous amount of gravitational energy is released therefore. For reasons of hydrostatic equilibrium a large part of this energy must be radiated away. As this does not show up in the luminosity of the central stars it must either leave them by some other means – e.g. neutrinos (Vila, 1966) – or it does not originate from the outset; in other words, the actual contraction must deviate strongly from the homologous way. I should like to discuss briefly the second possibility.