Results from simulated observations of HD 179949 b consisting of 100 spectra with an S/N_* of 200 each, with a R = 100 000 spectrograph covering the wavelength range λ = 2.32–2.45 μm. Left panel: cross-correlation function (CCF) using a pure ¹²C¹⁶O template spectrum on data set A (black solid line) which contains all species, and data set B (grey dashed line) which contains lines of ¹³C¹⁶O only. Data set A gives a signal with an S/N of ~13, while the main isotope, as expected, is not detected in data set B. Right panel: CCF using a pure ¹³C¹⁶O template spectrum on data set A (black dotted line) and data set B (solid blue line). In addition, we show the CCF using a noiseless telluric-free model for data set B as a template (red solid line). The ¹³C¹⁶O isotope is detected at an S/N of ~3.5 and ~5 in data set A and B, respectively. By using the perfect noiseless input model as a template, ¹³C¹⁶O’s detection S/N is further increased to an S/N of ~7. To reach the latter significance, the planet atmosphere needs to be well constrained.