1. Armandroff, T.E., and Da Costa, G.S.: 1991,Astron. J. 101, 1329.
2. Armandroff, T.E. and Zinn, R.: 1988,Astron. J. 96, 92.
3. Arp, H.C.: 1965, in A. Blaauw and M. Schmidt (eds.),Galactic structure, University of Chicago Press, p. 401.
4. Bell, R.A.: 1988, in J.E. Grindlay and A.G. Davis Philip (eds.),IAU Symposium 126, Kluwer, Dordrecht, p. 79.
5. Brown, J.A. and Wallerstein, G.: 1992,Astron. J. 104, 1818.