1. Quantitative classification of WR nuclei of planetary nebulae
2. Acker, A., Marcout, J., Ochsenbein, F., Stenholm, B., & Tylenda, R. 1992, Strasbourg - ESO catalogue of galactic planetary nebulae, Part 1, Part 2 (Garching: European Southern Observatory)
3. The formation and evolution of hydrogen-deficient post-AGB
white dwarfs: The emerging chemical profile and the expectations for
the PG 1159-DB-DQ evolutionary connection
4. Blöcker, T. 2003, in Planetary Nebulae: Their Evolution and Role in the Universe, ed. S. Kwok, M. Dopita, & R. Sutherland, IAU Symp., 209, 101