For some years now, there have been suggestions that the nuclei of planetary nebulae are stars undergoing a final gravitational contraction to the white dwarf state. These have culminated in two important studies by O’Dell, and by Seaton and his collaborators, the results of which are indicated on the Hertzsprung-Russell diagram of Figure 1. The temperatures and the distances—and hence luminosities—of the central stars are obtained from flux observations of the stars themselves and the surrounding nebular shells, the former by the Zanstra method, and the latter by that of Shklovsky.