Resonance amplification factor, W, vs. stellar angular velocity, Ω_s (solid red line). The vertical dashed green line indicates the value of the stellar angular velocity, Ω_er, which is stably maintained by the opposite effects of the stellar wind torque, acting to brake stellar rotation, and the value of theresonant tidal torque introduced in Sect. 3.8, acting to spin up the star when the orbital period of the planet is shorter than the stellar rotation period (see text).