Spectroscopy of the central stars of three evolved planetary nebulae

We present results of a spectroscopic project that aims at investigations of stars in the upper left part of the HRD. We observed the central stars of the three old planetary nebulae PN G011.4+17.9, PN G308.2+07.7, and PN G332.5-16.9. The spectra (echelle) were analysed by fitting the H and He II lines with theoretical profiles from a grid of non-LTE model atmospheres. The models are composed of H and He and disregard metal opacities. The three nuclei appear to be very hot (T(eff) near 10(5) K) and comparison with post-AGB evolutionary tracks reveals that the masses range between 0.55 and 0.75 M(.); the central star of PN G332.5 - 16.9 shows an enrichment in He and probably also in C and N.