Evolution of the inferred M_Co II ∕ M_Fe II ratio with time. We assumed a rise time of 18 days (Ganeshalingam et al. 2011). The error bars reflect the 68% posterior interval of the mass ratio. The coloured lines show the expected mass ratio M_Co ∕ M_Fe of the M_Ch delayed-detonation model “N100” (Seitenzahl et al. 2013, green), the sub-M_Ch CO detonation model “det_2010_1.06” (Sim et al. 2010, orange) and the M_Ch “W7 Z_⊙” model (Nomoto & Leung 2018, red). The black line is not a fit to the data and represents the M_Co ∕ M_Fe ratio assuming only radioactive decay from ⁵⁶Ni to ⁵⁶Co to ⁵⁶Fe. The same ionisation fractions of Co II and Fe II allow us to generalise M_Co II ∕ M_Fe II to M_Co ∕ M_Fe and compare the two ratios (see Sect. 5).