Ferrule for counter-rotating turbine impeller

A counter-rotating turbine for a turbine engine extending around an axis of rotation and comprising:

• an inner rotor configured to rotate around the axis of rotation, and comprising an inner drum to which a plurality of inner impellers is attached, each comprising inner moving blades and being rotatably supported by a first shaft,
• an outer rotor configured to rotate around the axis of rotation in a direction opposite to the direction of rotation of the inner rotor, and comprising an outer drum to which is attached a plurality of outer impellers, each comprising outer moving blades and being rotatably supported by a second shaft coaxial with the first shaft, the outer rotor comprising a downstream impeller which is downstream the other outer impellers according to the flow direction of the gases in the turbine along the axis of rotation, the downstream impeller having a plurality of downstream moving blades extending between an outer shroud and an inner shroud, an upstream end of the outer shroud being attached to the outer drum downstream of said outer drum, the inner shroud being attached to the second shaft,
• at least one of the inner shroud and of the outer shroud comprising at least one flexible transition part configured to allow elastic deformation of said shroud in the radial direction, wherein the outer shroud is attached to a platform of a radially outer end of a downstream moving blade, and wherein an upstream end of the platform is a free end not attached to the outer drum or to the outer shroud.