Quantitative evidence of an intrinsic luminosity spread in the Orion nebula cluster M. Reggiani, M. Robberto, N. Da Rio, M. R. Meyer, D. R. Soderblom and L. Ricci A&A, 534 (2011) A83 Published online: 07 October 2011 DOI: 10.1051/0004-6361/201116946