1. Abell, G. O.: 1966, Astrophys. J. 144, 259.
2. Aller, L. H.: 1943, Astrophys. J. 97, 135.
3. Aller, L. H.: 1956, Gaseous Nebulae, Chapman & Hall, London, p. 211.
4. Aller, L. H.: 1967, in D. E. Osterbrock and R. C. O’Dell (eds.), ‘Planetary Nebulae’, IAU Symposium No. 34, 339.
5. Aller, L. H. and Heap, S. R.: 1971, in preparation; Abstract in Bull. American Astron. Soc. 3, 400.