Fig. 2 – Stacked Fe II CCFs, in the Solar System’s barycentric rest frame, of the three transits, before (left) and after (right) removing a model of the RM and CLV effects from the spectra. The dash-dotted lines mark the beginning and end of transit (we started observing the transits of MASCARA-2 b shortly after they began). The slanted dashed lines with arrowheads trace the predicted radial velocity of the planet outside transit (Gaudi et al. 2017; Lund et al. 2017). While the RM and CLV contributions are evident as a bright yellow ‘Doppler shadow’ in the in-transit CCFs in the left panels of the MASCARA-2 b transits, they are not so obvious in the KELT-9 b CCFs. This is because KELT-9 b is on a polar orbit, and as we are using both in- and out-of-transit CCFs to calculate the master stellar CCF, the Doppler shadow largely vanishes. We note that the left panels are presented just for the purpose of visualising the RM and CLV correction: we correct the spectra prior to cross-correlation, so these contaminated CCFs are not part of our analysis. The atmospheric signal of Fe II from KELT-9 b is already discernible as a brown streak that traces the planet’s velocity in the in-transit CCFs.