Wavelength-dependent detection S/N for the secondary ¹³C¹⁶O (orange boxes) and the main ¹²C¹⁶O isotopologue (gray boxes). The box widths correspond to the wavelength range of the synthetic observations, while the height corresponds to the 16 and 84 percentiles of themeasured S/Ns, as derived by running the simulations multiple times. For ¹³C¹⁶O the S/N of data set B cross-correlated with the pure isotopologue template spectrum are shown, which is conservative, corresponding to the solid blue line in Fig. 3. For ¹²C¹⁶O the S/N of data set A cross-correlated with the pure isotopologue template spectrum are shown. We assumed 100 observationswith S/N_*(2.3 μm) = 200, used to calculate the stellar S/N as function of wavelength. In the background, we show the telluric transmission model (gray solid line), as well as the scaled and offset logarithm of the CO opacity at T = 1200 K and P = 10⁻⁴ bar (light red solid line). The CO detection by Brogi et al. (2014) is shown in cyan. The actual S/N value of the CO detection in Brogi et al. (2014) is 5.8, but due to the larger wavelength coverage of our bins one has to scale this value up to a S/N of 9. This is somewhat lower than our prediction for ¹²C¹⁶O, but the S/Nof their observations is also smaller.