1. L. Perek and L. Kohoutek, Catalog of Galactic Planetary Nebulae, Prague (1967).
2. I. S. Shklovskii, Astron. Zh.,33, 222 (1956).
3. M. Liller, B. Welther, and W. Liller, Astrophys. J.,144, 280 (1966).
4. O. N. Orlova, Astron. Zh.,49, 1164 (1972).
5. O. C. Wilson, Astrophys. J.,111, 279 (1950).