1. A. Acker, J. Köppen, B. Stenholm, and G. Jasniewicz, Astron. Astrophys. Suppl. Ser. 80, 201 (1989).
2. A. Acker, J. Marcout, F. Ochsenbein, B. Stenholm, and R. Tylenda, The Strasbourg-ESO Catalogue of Galactic Planetary Nebulae (European Southern Observ., 1992).
3. F. J. Ahern, Astrophys. J. 223, 901 (1978).
4. L. H. Aller, Astrophys. J. 93, 236 (1941).
5. L. H. Aller, Physics of Thermal Gaseous Nebulae (Reidel, Dordrecht, 1984).