Sinusoidal fit of the B2 streamline. Maser labels (see Table 1) mark the position of the masers of the B2 stream. 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. Red labels and arrows denote the positions and proper motions of a subset of masers showing similar velocities (see Fig. B.3). The black dashed line draws the sinusoid (see Table 2) fitted to the positions of the masers with black labels. The blue labels indicate a few masers scattered in position with respect to the sinusoid.