1. Aanestad P.: 1973,Astrophys. J. Suppl. 25, 223.
2. Allen, C. W.: 1973,Astrophysical Quantities, 3rd ed., London: University of Longon, The Athlone Press, p. 30.
3. Bell, K. L. and Kingston, A. E.: 1967,Monthly Notices Roy. Astron. Soc. 136, 24.
4. Berkhuijsen, E. M.: 1971,Astron. Astrophys. 14, 359.
5. Berkhuijsen, E. M.: 1972,Astron. Astrophys. Suppl. 5, 263.