French et al. (2007) HST measurements, A₄₀ (upper frame) and ∆θ_min (lower frame) plotted as a function of τ_pps. The measurements (both B_eff = 10° and 15°) are colour-coded according to the ring regions B1-B4 and A1-A2 defined in Colwell et al. (2009); see the insert frames. Thick dashed lines indicate B = B′ = 12° photometric models with W = 20 size distribution, for τ_dyn = 0.75, 1.0 and 2.2. The thin dashed line in the upper frame corresponds to simple estimate A₄₀ = 0.3T /(1 − T) discussed in the text.