Early emission of rising optical afterglows: the case of GRB 060904B and GRB 070420 A. Klotz, B. Gendre, G. Stratta, A. Galli, A. Corsi, B. Preger, S. Cutini, A. Pélangeon, J. L. Atteia, M. Boër and L. Piro A&A, 483 3 (2008) 847-855 Published online: 11 March 2008 DOI: 10.1051/0004-6361:20078677