1. Alexander, D. R.: 1975,Astrophys. J. Suppl. Ser. 29, 363.
2. Arkhipova, V. P.: 1982,Perem. Zvezdy, Byull. (in press).
3. Bertraud, C.: 1961,J. Obs. 44, 81.
4. Cox, A. N. and Tabor, J. E.: 1976,Astrophys. J. Suppl. Ser. 31, 271.
5. Cox, J. P., King, D. S., Cox, A. N., Wheeler, J. C., Hansen, C. J., and Hodson, S. W.: 1980,Space Sci. Rev. 27, 529.