Top: radio light curve at 2 (red circles) and 8 GHz (blue squares) observed by the GBI from MJD 50 154.6 until 50 175.7 (orbital phase Φ = 0.1−1.0), averaged over one day. Bottom: radio spectral index α of the radio fluxes shown in the top panel clearly follows a two-peaked trend. One peak is during the large apastron radio outburst, which features a flat or inverted (α ≥ 0) spectrum during its rise. The other peak of the spectral index occurs at Φ = 0.25, i.e. at periastron, revealing that the spectrum becomes flattish (α ≈ 0) during this orbital phase as well (Massi & Kaufman Bernadó 2009).