Zoom-in of the GTC/OSIRIS optical spectra of W1738 in the region dominated by methane (and possibly hydrogen sulphide) and relatively free from telluric absorption. The spectra obtained at six different epochs (labelled) are compared with the co-added spectrum (solid black, and the part below 8300 Å was co-added using only the spectra in 2021). In 2014, W1738 had more flux in the i-band than in 2021 because the Y dwarf passed very close to a background galaxy that contaminated blue part of the spectrum.