Transmission spectroscopy of KELT-10 b in the region of the sodium doublet. In gray, the entire transmission spectrum is shown (corrected for RM effect, as described in Sect. 5.2), with 20x bins in dark blue. The detection of McCloat et al. (2021) is overplotted (dark red) as a visual cue for comparison. The black dashed line corresponds to the 3σ and 5σ limit above the continuum $\left(R_{\text{p}}^2/R_{\text{s}}^2\sim 1.44\%\right)$. The yellow area is the ±1.5 Å region around each of the Na I lines.