Viscous and front propagation timescales t_visc and t_front evaluated at R_sh as a function of orbital eccentricity. The upper (green) horizontal shaded area represents the period of the shock-induced perturbation (equal to that of the QPE or QPO, i.e. to the orbital period in the assumed scenario), while the lower (magenta) one is the shortest observed time delay of the QPO with respect to the immediately preceding QPE (XMM3).