  Based on our analysis of the contributions from the connected and
disconnected contraction diagrams to the pion-kaon scattering amplitude, we
provide the first determination of the low-energy constant $L_0^r$ in SU$(4|1)$
Partially-Quenched Chiral Perturbation Theory from data of the Extended Twisted
Mass Collaboration, $L_0^r = 0.51(26)\cdot 10^{-3}$ at $\mu=1$ GeV.
