Dust SEDs for E- and B-mode polarization derived from the SMICA component-separation procedure (Planck Collaboration IV 2020). The two grey lines represent MBB fits to the E- (red diamonds) and B-mode (blue squares) data points with a temperature of 19.6 K. The polarization spectral index derived from the fits is  = 1.53 ± 0.02. The residuals to each fit, normalized to the 1 σ data uncertainty, are plotted in the lower panel.