Boller et al. (1997);
Page et al. (2001);
Hibbard & van Gorkom (1996);
Smith et al. (1996);
Vorontsov-Velyaminov & Arhipova (1968);
Rodriguez Espinosa et al. (1987);
Risaliti et al. (2000);
Gonzalez-Delgado et al. (1997);
Bonatto et al. (1989);
Stauffer (1982);
Ho et al. (1995);
Alonso-Herrero et al. (2000);
Sharples et al. (1984).