1. Acton, L. W., Culhane, J. L., Gabriel, A. H., Wolfson, C. J., Rapley, C. G., Phillips, K. J. H., and 12 co-authors: 1981, Astrophys. J. 244, L137.
2. Chiuderi, C.: 1981, in R. M. Bonnet and A. K. Dupree (eds.), Solar Phenomena in Stars and Stellar Systems, D. Reidel Publ. Co., Dordrecht, Holland, p. 267.
3. Chiuderi, C.: 1982, in J. O. Stenflo (ed.), Solar and Stellar Magnetic Fields: Origins and Coronal Effects, D. Reidel Publ. Co., Dordrecht, Holland, p. 375.
4. Cowling, T. G.: 1981, Ann. Rev. Astrophys. 19, 115.
5. Ferrari, A., Rosner, R., and Vaiana, G. S.: 1982, Astrophys. 263, 944.