Linearly polarized spectrum of Betelgeuse (with the continuum polarization level set to zero) around the Na i lines D2 (588.9 nm) and D1 (589.6 nm) for Set 1 (left) and Set 2 (right). In addition a blend of lines of Ni i and Fe i (at 589.3 nm, between D2 and D1) and a line of Ti i at 589.9 nm are visible. The upper spectrum shows Stokes I and the lower spectra show Stokes Q (blue) and Stokes U (red). The vertical lines correspond to the position of the Na i D lines in the stellar rest frame.