1. Burbidge, E. M. & Sargent, W.: 1970, InNuclei of Galaxies, ed. D. O'Connell, (North Holland), p. 351
2. Hickson, P.: 1982,ApJ,255, 382
3. Hickson, P. Kindl, E. & Huchra, J.: 1988,ApJ Lett.,329, L65
4. Hickson, P., Mendes de Oliveira, C., Huchra, J. & Palumbo, G.: 1992,ApJ,399, 353
5. Karachentseva, V. et al.: 1979,Astroph. Issled. 11, 3