Clustering of the optical-afterglow luminosities of long gamma-ray bursts M. Nardini, G. Ghisellini, G. Ghirlanda, F. Tavecchio, C. Firmani and D. Lazzati A&A, 451 3 (2006) 821-833 Published online: 04 May 2006 DOI: 10.1051/0004-6361:20054085