Four-month chromospheric and coronal activity cycle in τ Boötis M. Mittag, J. Robrade, J. H. M. M. Schmitt, A. Hempelmann, J. N. González-Pérez and K.-P. Schröder A&A, 600 (2017) A119 Published online: 12 April 2017 DOI: 10.1051/0004-6361/201629156