Detection of ¹³CO in the atmosphere of β Pic b. Panel a: combined observed spectrum from the six nights with S/N > 5 in the ¹³CO overtone bands, shifted to the planet rest-frame and binned to 10 km s^–1 for display. The two models are averages of the individual best-fit models with and without ¹³CO. Panel b: residuals between the observed spectrum and the model without ¹³CO, with a ¹³CO-only model overplotted. Panel c: cross-correlation of the residuals, averaged over the full wavelength range and the six nights, with a ¹³CO template. The dashed line shows the template auto-correlation function, and the faint grey lines show the individual-night cross-correlations.