Calibrated and telluric corrected GTC/OSIRIS spectrum of W1738. In total, six-epoch spectra in 2014 and 2021 were co-added, but the part bluer than 8300 Å was co-added using only three epochs in 2021 because in 2014 the object was contaminated by a background galaxy. The spectrum in light grey is dominated by telluric correction noise. Full transmission of the GTC/HiPERCAM i and ɀ filters, including atmosphere, optics, and CCD response, are overplotted.