Top panel: MUSE one-dimensional spectra of the quasar (QSO) image B (S1 in black, rescaled by a factor of 7) and the hidden galaxy GX lying close to it, highly contaminated by the QSO image C (S2 in red). Both spectra are extracted from circular apertures of 0.″3 radius, indicated in red in the HST F814W image cutout, where QSO B and C have been subtracted. Bottom panel: two portions of the subtracted one-dimensional spectrum (S2−S1), where the [O II] and [O III] emission lines of a source at a redshift of 0.6735 are highlighted.