1. Auer, L. H., Heasley, J. N. and House, L. L.: 1977,Astrophys. J. 216, 531.
2. Brown, J. C. and McLean, I. S.: 1977,Astron. Astrophys. 55, 347.
3. Brown, J. C., McLean, I. S. and Emslie, A. G.: 1978,Astron. Astrophys. 68, 415.
4. Buerger, P. F. and Collins, G. W.: 1970,Astrophys. J. 161, 1025.
5. Chandrasekhar, S.: 1960,Radiative Transfer, Dover, New York.