Best fit model of SN 2013aa at 378 d in the H-band. Shown is the extinction and redshift corrected spectrum (black line) and individual contributions from the ions Fe II (orange), Co II (green) and Co III (red) as solid curves. The purple line is the combined emission of the three ions. The two solid vertical lines are used to compute M_Co II ∕ M_Fe II. Dashed curves indicate the contribution of the individual lines in the blended region. We do not show Ni II as it does not have any lines in this region.