Top left panel: Mrk 231 total flux as a function of wavelength. Top right panel: Polarized flux as a function of wavelength, established from the multiplication of the flux and the polarization degree. Bottom left panel: Degree of polarization as a function of wavelength. Bottom right panel: Polarization position angle minus the radio position angle (5°, see Ulvestad et al. 1999) as a function of wavelength. Below each panel, we give the residuals between the observational data and the theoretical model, plotted across the wavelength range.