Close-ups on the carbon and helium lines used to constrain the luminosity ratio (solid blue) compared to the combined synthetic model (dotted red). Left: observed FUSE spectra (ID 01). The weighted synthetic spectra of the primary and secondary components are shown as a dotted black and green line. Right: observed X-shooter spectra (ID 09 and 31). The unweighted synthetic spectra of the primary and secondary components are shown as a dotted black and green line.