Chromospheric activity and evolutionary age of the Sun and four solar twins M. Mittag, K.-P. Schröder, A. Hempelmann, J. N. González-Pérez and J. H. M. M. Schmitt A&A, 591 (2016) A89 Published online: 20 June 2016 DOI: 10.1051/0004-6361/201527542