1. J. Heidman and A. T. Kalloglyan, Astrofizika,9, 71 (1973).
2. J. Heidman and A. T. Kalloglyan, Astrofizika,11, 229 (1975).
3. C. Casini and J. Heidman, Astron. Astrophys.,39, 127 (1975).
4. C. Casini and J. Heidman, Astron. Astrophys.,47, 371 (1976).
5. E. Ye. Khachikian, Astron. Nachr.,297, 287 (1976).