Elliptic orbit of a low-mass companion (blue) through the AGB dust-forming region (red). Orbital-plane frame (X_p, Y_p, Z_p) (dashed) and focal frame (X, Y, Z) (solid) are shown. The position vector r from M₁ to M₂ is set by Ω (longitude of the ascending node), i (inclination), ω (argument of periastron; here 225°)¹, a (semi-major axis), e (eccentricity), and T₀ (time of periastron passage), alongside the true anomaly, ν. The companion and its dusty wake cross the spherical dust zone (red dotted) over an apastron-centred phase interval.