In Section 4.4 of [1] segment 2–3 of the outer rotor profile is given by Eq. (42). For β=0 this equation is equivalent to the statement that the spherical centre of the inner rotor circle moves along an arc of a great circle passing through the axis of the outer rotor. The same motion is specified in the paper by Eq. (29): the centre traces a spherical hypocycloid, all points of which but two lie off the plane of that great circle. The deviation is obtained in closed form; its maximum equals δ2−2δ1.