Photocentre shifts as predicted by the superposition of two Gaussian sources with different distance, flux ratio, and position angle (see text for details). The full and dashed lines correspond to PAs of 0° and 30°, respectively. The blue, green, and orange curves are for source distances of 70, 140, and 210 mas, respectively. The full rectangle indicates the range (mean ±1σ) of shifts in the blue components of the ¹³CO(1−0) lines in 2010.