1. Allen, D. A.: 1973,Monthly Notices Roy. Astron. Soc. 161, 145.
2. Andrillat, Y. and Fehrenbach, Ch.: 1982,Astron. Astrophys. Suppl. 48, 93.
3. Arnulf, A., Barbier, D., Chalonge, D., and Jafir, H.: 1938,Ann. Astrophys. 1, 293.
4. Ashok, N. M., Bhatt, H. C., Kulkarni, P. V., and Joshi, S. C.: 1984,Monthly Notices Roy. Astron. Soc. 211, 471.
5. Baldwin, R. B.: 1942,Sky Telesc. 1, 5.