(Left): Division of the BCDE1 and BCDE2 mask apertures into the sub-apertures BC1-DE1 and BC2-DE2. (Right): Comparison of the 1D spectra extracted from the BC1,2 and DE1,2 mask apertures. The latter were rescaled (by a factor of 1.39) in order to match the average F_λ of BC1,2 in the observed wavelength range 3.7 to 3.8 μm. This interval was chosen to highlight possible differences between the spectra in the UV slope (blueward of 3.7 μm) or in the Balmer break amplitude (redward of 3.8 μm).