Regulating system for a timepiece movement

An adjustment device for a regulating system for a timepiece movement,

• the regulating system comprising:
• a frame,
• an assembled balance pivoted in relation to the frame about a geometric axis,
• an elastic return system adapted to link the assembled balance to the frame so that the assembled balance and the elastic return system form an oscillator,
• the elastic return system comprising:
• a first elastic return element forming a first hairspring having a first stiffness k1,
• a second elastic return element having a second stiffness k2,
• a third elastic return element having a third stiffness k3, and
• a device for modifying the third stiffness k3,
• the first elastic return element and the second elastic return element being assembled in series between the assembled balance and the frame, and
• the third elastic return element and the second elastic return element being assembled in parallel between the frame and the first elastic return element,
• wherein the adjustment device is adapted for modifying the third stiffness k3 of the third elastic return element.