1. Amoruso, F.: 1975, unpublished doctoral thesis, University of Padua.
2. Blaauw, A.: 1965, inStars and Stellar Systems, Vol. V, p. 435.
3. Brosche, P.: 1970,Astron. Astrophys. 6, 240.
4. Eggen, O. J., Lynden-Bell, D. and Sandage, A.: 1962,Astrophys. J. 136, 748.
5. Gott, J. R. and Thuan, T. X.: 1976,Astrophys. J. 204, 649.