1. Aller, H. D., Aller, M. F., Latimer, G. E., and Hodge, P. E.: 1985,Astrophys. J. Suppl. 59, 513.
2. Beskin, G. M., Lyutyi, V. M., Neizvestnyi, S., Pustil'nik, S. A., and Shvartsman, V. F.: 1985,Soviet Astron. 29, 252.
3. Bortle, J.: 1983a,IAU Circ. No. 3780.
4. Bortle, J.: 1983b,IAU Circ. No. 3792.
5. Brindle, C., Hough, J. H., Bailey, J. A., Axon, D. J., and Hyland, A. R.: 1986,Monthly Notices Roy. Astron. Soc. 221, 739.