1. I. I. Romanyuk, Bull. Spec. Astrophys. Obs. 58, 63 (2005).
2. P. Renson, R. Farragiana, and F. Catalano, Astronom. and Astrophys. Suppl. Ser. 89, 429 (1991).
3. P. Renson, D. Kobi, and P. North, Astronom. and Astrophys. Suppl. Ser. 89, 61 (1991).
4. G. W. Preston, Ann. Rev. Astronom. Astrophys. (Palo Alto, Calif., 1974), p.257.
5. E. F. Borra, J. D. Landstreet, and L. Mestel, Ann. Rev. Astronom. Astrophys. 20, 191 (1982).