1. Properties of Some Old Planetary Nebulae
2. Quantitative classification of WR nuclei of planetary nebulae
3. Acker, A., Gleizes, F., Chopinet, M., et al. 1982, CDSSP, 3
4. Acker, A., Marcout, J., Ochsenbein, F., Stenholm, B., & Tylenda, R. 1992, Strasbourg-ESO catalogue of galactic planetary nebulae (Garching: European Southern Observatory)
5. The Absorption-Line Spectra of the Central Stars of the Planetary Nebulae.