1. Dravskikh, Z.V., and Dravskikh, A.F.: 1964, Astron. Tsirk., 158, 2
2. Hill, J.K.: 1977, Astrophys. J.
212, 692
3. Hoang-Binh, D., Walmsley, C.M.: 1974, Astron. Astrophys.
35, 49
4. Gorkom, J. van: 1980, Ph.D. Thesis, University of Groningen, The Netherlands.
5. Kardashev, N.S.: 1959, Soviet Astr., 3, 813