As mentioned previously, we neglect the reverse SF process, triplet fusion, in this model. This assumption is justified in TIPS-Pn, as fusion is energetically uphill. For both TIPS-Pn and TIPS-Tn, the model fits well without triplet fusion, and so presumably it plays only a minor role. The other assumption made here is that the ISC rate is negligible compared to SF, but, as we show below, this assumption may not necessarily be the case.