Stellar rotation periods determined from simultaneously measured Ca II H&K and Ca II IRT lines M. Mittag, A. Hempelmann, J. H. M. M. Schmitt, B. Fuhrmeister, J. N. González-Pérez and K.-P. Schröder A&A, 607 (2017) A87 Published online: 20 November 2017 DOI: 10.1051/0004-6361/201630262