Mass-radius relations for the polytropic-quark EOS with realistic SLy4 crust. Selected M(R) relations correspond to a polytropic segment of γ = 4.5, connected to the SLy4 crust at n₀ = 0.21 fm⁻³ and to a linear EOS of α = 1 (Eq. (2)) at n₁ = 0.335 fm⁻³. The curves differ by the density jump λ = n₂/n₁, as indicated in the plot. Regions destabilised by the phase transition are indicated by dotted segments. The black line indicates the SLy4 EOS. The pale green and blue bands correspond to the mass ranges estimated in the low-spin prior case of GW170817 (Abbott et al. 2017).