A casing (1) with a circular cylindrical internal surface (3) delimits an operating chamber. A rotor (4) orbits about a chamber axis which is the axis of the internal surface (3). The rotor (4) has a circular cylindrical external surface (11), a generatrix of which is adjacent to the internal surface (3), a diametrically opposite genetrix being spaced from the internal surface (3). A vane member (17), mounted on the casing (1) and pivotable about a pivot axis parallel to the chamber axis, is accommodated in a fluid inlet/outlet aperture (18) in the casing, the vane member having a passageway (17a) communicating between the exterior of the casing and the operating chamber. The vane member (17) has an arcuate face (17b) coaxial with the pivot axis, end faces (17b) towards the pivot axis, end faces (17c) extending from the respective lateral ends of the arcuate face (17b) towards the pivot axis, and a tip face (17g) adjacent the rotor (4), these faces (17b, 17c, 17g) being sealing faces with respect to corresponding surfaces of the aperture (18) and the rotor (4). A linkage (28) connects the vane member (17) to the rotor (4) so as to keep the tip face (17g) in sealing contact with the external surface (11) of the rotor, the linkage being connected to the vane member by an articulation having an articulation axis (30) such that a plane containing the articulation axis (30) and the axis of the external surface (11) passes through the region of sealing contact.