1. Altenhoff, W. J., Downes, D., Goad, L., Maxwell, A., Rinehart, R.: Astron. Astrophys., Suppl. 1, 319 (1970).
2. Ball, J. A., Gottlieb, C. A., Lilley, A. E., Radford, H. E.: Astrophys. J. 162, L203 (1970).
3. Brezgunov, V. N., Daghesamansky, R. D., Udal’tsov, V. A.: Astrophys. Letters, 9,117 (1971).
4. Burbidge, E. M., Burbidge, G. R.: Astrophys. J. 135, 694 (1962).
5. Cameron, M. J.: Observatory 88, 254 (1968).