The 2.35 year itch of Cygnus OB2 #9 - I. Optical and X-ray monitoringY. Nazé, L. Mahy, Y. Damerdji, H. A. Kobulnicky, J. M. Pittard, E. R. Parkin, O. Absil and R. BlommeA&A, 546 (2012) A37DOI: https://doi.org/10.1051/0004-6361/201219442
