HARPS sodium doublet transmission spectrum of WASP-166b for all nights combined shown in the PRF. Upper panel: grey, data points at full HARPS resolution. In black, grey data points binned by ×25 for visibility. The theoretical line centres for the sodium doublets are shown as vertical blue dashed lines; a Gaussian fit to the unbinned data is shown in red. The sodium absorption is visible for both lines of the doublet. The combined line contrast is measured at 0.455 ± 0.135%, resulting in a 3.4σ detection (see Sect. 4.3). Lower panel: residuals of the Gaussian fit in percent.