Step-by-step explanation of the position angle determination: spectra are recorded for different slit angles. Top: the 2D spectrum of one absorption line, tilted as a function of ψ. The tilts are exaggerated here in order to be visible. Middle: the position spectrum measured around the absorption line presenting the typical displacement of stellar rotation. Bottom: using the maxima of the displacement on the position spectrum, we fit a sine function whose phase is the stellar position angle. Here PA_star was set at 12°.