these lines as well as cosmic ray strikes and the general background, the data were processed using our own scripts to subtract the residual background along each column and interpolate across isolated pixels with highly anomalous counts. Care was taken to ensure that the scripts removed only noise features, using a comparison of the many image slices within each dispersed image to distinguish real features from noise.

We used the IRAF task [continuum]{} to normalise each science spectrum and remove the slope. We then fitted a second order chebyshev function to define a local pseudo-continuum. This pseudo-continuum set the flux level for measuring the equivalent widths of spectral features. While we experimented with higher-order fits, none differed significantly from the second-order fit. For this reason, the flux level of the pseudo-continuum was excluded as a source of error when we came to determine the uncertainties in our measurements. To obtain a representative pseudo-continuum, the fit excluded the NaI lines and the CO absorption bands starting at 2.29$\mu$m. Generally, the wavelength range for the fit was from 2.1$\mu$m $\rightarrow$ 2.2$\mu$m, and 2.21$\mu$m $\rightarrow$ 2.29$\mu$m. As a pseudo-continuum was used to measure these equivalent widths, they are more accurately referred to as 