Auto-resonance in CoRoT-4. Top panel: amplitude of the azimuthal oscillations of the resonant flux tube vs. the time (solid black line). The amplitude of the asymptotic solution (44) is plotted as a solid red line for A₀ = 0, and the amplitude in the case of a perfect and continuous phase locking (sin ψ = 1, constant) is plotted as a three-dot-dashed green line. Lower panel: sine of the phase lag angle ψ vs. the time in CoRoT-4.