Multi-wavelength spectroscopic analysis of the ULX Holmberg II X-1 and its nebula suggests the presence of a heavy black hole accreting from a B-type donor
S. Reyero Serantes, L. Oskinova, W. -R. Hamann, V. M. A. Gómez-González, H. Todt, D. Pauli, R. Soria, D. R. Gies, J. M. Torrejón, T. Bulik, V. Ramachandran, A. A. C. Sander, E. Bozzo and J. Poutanen
