Sinusoidal fits of the R1 and B1 streamlines. Maser labels (see Table 1) mark the position of the masers in the R1 (left panel) and B1 (right) streamlines. We warn that the radial positions are stretched out to allow one to read the maser labels. Positional errors are not shown since they are smaller than the labels. In both panels, the black dashed line draws the sinusoid (see Table 2) fitted to the maser positions.