The mid-infrared extinction law in the darkest cores of the Pipe Nebula J. Ascenso, C. J. Lada, J. Alves, C. G. Román-Zúñiga and M. Lombardi A&A, 549 (2013) A135 Published online: 15 January 2013 DOI: 10.1051/0004-6361/201220658