GTC/OSIRIS optical spectrum of W1738, co-added with the spectrum by Leggett et al. (2016) in the range 10 000–10 900 Å, compared with theoretical spectra of different temperatures from Morley et al. (2014). The best overall agreement between the observed spectrum and the theoretical spectrum is for an effective temperature of 400 K. The lower panel is the same as the upper panel but with a log scale in the y-axis to enhance the low-flux region.