Left panel: comparison of Hγ profiles from two VLT-FLAMES epochs in the rest frame of the narrow-lined B-type star. At these epochs the star is close to each quadrature phase. An arrow indicates the extra absorption that we attribute to the presence of a third star, not present in the blue spectrum. If this component was due to a faint companion to the B-type star then it should be visible on the red wing of the blue spectrum. Instead its velocity at that epoch is almost identical to that of the B-type star, see Sect. 3.2, hence no absorption is apparent at that epoch. Right panel: as for the left panel except here we plot two VLT-FLAMES epochs of the He I 4471 Å line. Similar considerations apply.