Differential phases φ_diff(λ) in the H_α line for different baseline orientations. Top: stellar disk deformed by the fast rotation and projected onto the sky where we have represented the baseline orientations: 0° (cross), 45° (star), 90° (solid line) and 160° (diamond). Bottom: evolution of φ_diff(λ) according to the baseline orientations represented on top and computed for a sixty meter-baseline. In this example we assumed that the position angle of the stellar rotation axis is equal to zero.