Long-term optical variability of high-mass X-ray binaries - II. Spectroscopy P. Reig, A. Nersesian, A. Zezas, L. Gkouvelis and M. J. Coe A&A, 590 (2016) A122 Published online: 26 May 2016 DOI: 10.1051/0004-6361/201628271