Magnetic field strength along the line of sight versus the radial distance of the masers to the star. The black boxes show typical regions of the plot where results from the literature for SiO, H₂O, and OH maser occur, and they are normalized for R_∗ = 1 (Vlemmings et al. 2002, 2005; Herpin et al. 2006; Rudnitski et al. 2010). Our measurements are shown by the hollow blue circles (IK Tau), hollow green squares (RT Vir), and filled red triangles (IRC+60370). The short-dashed, solid, and long-dashed inclined lines show a dependence ∝ R^-1, ∝ R^-2, and ∝ R^-3 for the magnetic field, respectively. The position of the AGB surface of a star with radius of 1 AU is also shown.