Velocity-integrated ¹²CO J = 2−1 line emission is shown in the inset, superimposed over the optical image obtained with the Hubble Space Telescope (Balick et al. 1997). Only the brightness distribution above 60 mJy beam^-1 (which corresponds to  ~8σ, where σ is the root-mean-square noise) was included to better separate the close projected image of the two rings. At the center the continuum emission is represented by a red dot, which traces the position of the stellar system. More details can be found in Sect. 2.