GTC-OSIRIS spectra of SDSS J1339+1310. These are derived from a 2.7 ks exposure on 13 April 2013. The red, blue, and green lines trace the fluxes of A, B, and G (see Fig. 2) as a function of observed wavelength. Data from frames in the r (circles) and I (square) bands are also shown for comparison purposes. The dashed lines describe the responses of the two filters, i.e., the LT r-Sloan and the NOT #12 passbands. Vertical dotted lines are associated with emission lines at z_s = 2.24, while grey highlighted regions indicate atmospheric artefacts (see main text).