CO model (solid lines) for the S-type AGB-star χ Cyg overlaid on observations (histograms and points). Models have been scaled by the indicated factor (obtained from requiring that the velocity-integrated intensities are equal to the observed ones). Also shown are derived brightness distributions (solid lines) overlaid on interferometric observations using the PdBI (blue points from Castro-Carrizo et al. 2010). A ¹²CO/¹³CO ratio of 43 was used.