Ni and Co masses in M_⊙ calculated from the strength of forbidden emission lines via Eq. (3). Open symbols: Ni-masses from the [Ni ii] 6.63 μ feature; filled symbols: Co-masses from the [Co ii] 10.52 μ feature. Lines show the expected amount of Co predicted from the radioactive decay of 0.02 M_⊙ radioactive ⁵⁶Ni synthesized in the explosion. Continuous line: only ⁵⁶Co; dashed line: ⁵⁶Co plus ⁵⁷Co with solar abundance ratio; dotted line: ⁵⁶Co plus ⁵⁷Co assuming twice solar abundance ratio.