Position-velocity diagrams of the ¹²CO J = 2−1 line emission in M 2−9 along the east-west (on the top) and north-south (on the bottom) directions. The green (violet) dotted lines represent the kinematical and spatial center of the inner (outer) ring. Top: integrated brightness from the whole nebula. Bottom: brightness obtained within a slit along the projected symmetry axis. With a green (violet) solid line we plot the position-velocity gradient corresponding to the entire inner (outer) ring along the north-south direction. A white contour traces the emission integrated over the complete nebula at low brightness level. Note that the (V_LSR) velocity axis shows the expanding gas speed projected on the line of sight. A more detailed discussion can be found in Sect. 3.2.