1. Adams, W. M. and Tang, F.: 1977, Solar Phys. 55, 499.
2. Antonucci, E.: 1978, in G. Belvedere and L. Paterno (eds.), Proc. Workshop on Solar Rotation, Publ. Catania Obs. No. 162, p. 204.
3. Bruzek, A.: 1961, Z. Astrophys. 51, 75.
4. d'Azambuja, L. and d'Azambuja, M.: 1948, Ann. Obs. Paris, Meudon 6, Part 7.
5. Durney, B. R., Keil, S. L., and Lytle, D. M.: 1984, Astrophys. J. 281, 455.