Position-velocity diagram of the CO(2–1) emission, as traced by ALMA observations, along an 0.4′′-wide 100′′-long SE-NW oriented slit (PA = 45°) passing through the star. The dense shells seen in Fig. 7 appear as bright elliptical arcs. The white ellipses correspond to shells of radii 16′′, 32′′, 42′′, 48′′, centred within 2′′ from the continuum peak and in radial expansion at a velocity of 14.5 km s^-1.