1. Acker, A., Ochsenbein, F., Stenholm, B., Tylenda, R., Marcout, J. & Schohn, C. 1992, Strasbourg-ESO Catalogue of Galactic Planetary Nebulae (Garching: ESO)
2. Aller, L. H. 1984, Physics of Thermal Gaseous Nebulae (Dordrecht: Reidel)
3. Chemical compositions of planetary nebulae
4. Aller, L. H. & Liller, W. 1968, in Nebulae and Interstellar Matter, ed. B. M. Middlehurst & L. H. Aller (Chicago: Univ. Chicago Press), 498
5. Asplund, M. 2003, in ASP Conf. Ser. 304, CNO in the Universe, ed. C. Charbonnel, D. Schaerer, & G. Meynet (San Francisco: ASP), 275