Comment on ''Exact analytical form for the box diagram with one heavy external quark''

Within the framework of the electroweak standard model, I recalculate an exact analytical form of the effective Hamiltonian for the box-diagram amplitude with one heavy external quark. For the absorptive part of the effective Hamiltonian, my result includes all possible thresholds, compared to the result of He, McKellar, and Pallaghy where only the lowest threshold was included. The analyses of mixing and CP violation in the neutral Tu0- and Tc0-meson systems are also discussed