Panel (a): the absolute value of the tidal potential Love number k₂; panel (b): the logarithm of the tidal quality function ∣K₂∣ = k₂/Q₂ as a function of the physical frequency χ for the three-layered models of Pluto and Charon presented in Table 4. The reference frequency is χ₀ = 1 rad s⁻¹. The vertical lines indicate the frequencies relevant at the beginning of the evolution (gray dotted line, similar values for both bodies) and for the spin-orbital configuration after 1 Myr (light-blue and pink dotted lines for Pluto and Charon, respectively) in the case with Ω₀/n₀ = 50 (Case 1, Table 3). They serve as an illustration of the typical tidal frequencies rather than an overview of all frequencies used.