Radio fluxes and spectral indexes around the periastron passage of Cyg OB2 #9, as a function of orbital phase. The top figure shows the 6 cm flux, the middle one the 20 cm flux, and the bottom one the spectral index. The older VLA data from van Loo et al. (2008) are plotted with a grey line. The new EVLA data are shown with the thicker solid red line. The length of the line indicates the 1σ error bar on the flux. For the non-detections, upper limits are shown that are 3 × the rms noise in the centre of the image. The fluxes have an additional uncertainty of ~5% due to the absolute flux calibration (not shown on the figure). On the spectral index figure, the dashed line indicates the value for free-free emission (α =  + 0.6). Periastron occurs at phase = 0.0.