The difference in age of the two counter-rotating stellar disks of the spiral galaxy NGC 4138 A. Pizzella, L. Morelli, E. M. Corsini, E. Dalla Bontà, L. Coccato and G. Sanjana A&A, 570 (2014) A79 Published online: 21 October 2014 DOI: 10.1051/0004-6361/201424746