SpeX/SXD near-infrared spectrum of TOI-2406 (black line) compared to equivalent data for the M4.0 spectral standard Gliese 213 (data from Rayner et al. 2009; magenta line). Top: full spectra, with the uncertainty spectrum of TOI-2406 is shown in black along the bottom, and regions of strong telluric absorption are indicated by the grey panels. Bottom: close-up of the K-band region for these two spectra, highlighting the metallicity-sensitive absorption features Na I (λ2.2079 μm) and Ca I (λ2.2640 μm), as well as CO band heads. The difference between the spectra (blue line) is consistent with the measurement uncertainties (grey band).