Polarisation spectra of SN 2021irp before and after the ISP subtraction. (a) Total polarisation P, Stokes parameters Q and U, polarisation angle θ, and S/N before the ISP subtraction at a phase of 203.0 d (black lines). The data are binned to 50 Å per point. The grey lines in the background of each plot are the unbinned total-flux spectra at the same epoch. The ISP is described by P_ISP = 0.32 %, θ_ISP = 8.8°(red lines). The blue hatching shows the adopted wavelength range for the ISP-dominated components. (b) Same as (a), but after the ISP subtraction. The brown and blue lines show the transmission curves of the R band and FILT_815_13. The red hatching shows the adopted wavelength range for the estimate of the continuum polarisation.