Observational properties of massive black hole binary progenitors R. Hainich, L. M. Oskinova, T. Shenar, P. Marchant, J. J. Eldridge, A. A. C. Sander, W.-R. Hamann, N. Langer and H. Todt A&A, 609 (2018) A94 Published online: 18 January 2018 DOI: 10.1051/0004-6361/201731449