Luminosity-dependent changes of the cyclotron line energy and spectral hardness in Cepheus X-4 V. Vybornov, D. Klochkov, M. Gornostaev, K. Postnov, E. Sokolova-Lapa, R. Staubert, K. Pottschmidt and A. Santangelo A&A, 601 (2017) A126 Published online: 19 May 2017 DOI: 10.1051/0004-6361/201630275