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line (B1). The binary is very close (B2), such that it is unresolved by the telescope. The diffracted image thus formed (B3) has no spatial information that would reveal the binary nature of the object. After passing through the slit and imaged in the detector and fitted (B4), the resultant spectro-astrometric signal is presented (B5). Several observations can be made (B5):

1. The signal is in between the two stellar positions. This is expected from a barycenter.

2. The amplitude of the spectro-astrometric signal is smaller than the separation of the binary. Only in a very special case, where one of the components would have negligible flux with respect to the other, and vice-versa, should the amplitude be equal to the separation.

3. In the spectral region of the emission line of the top component, the spectro-astrometric signal shifts towards the top, and in the spectral region of the absorption line of the bottom component the signal shifts to the bottom.

The interesting feature of the results we have just obtained is that we know that small-scale structure is present in the object and that this small structure could not be found by simply inspecting the diffracted image B3. The difficulty is now in interpreting and quantifying the spectro-astrometric signal in terms of actual object properties. Some of the more interesting results obtained to date with spectro-astrometry are described in Sect. 3.

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