higher resolution, which makes it possible to discard these objects or treat them appropriately.

For systems which are unresolved also by [*Gaia*]{} the space motions of their photocentres will deviate from the linear uniform model represented by the astrometric parameters. In the later astrometric solutions of [*Gaia*]{} data, objects that do not fit the single-star astrometric model will be detected and filtered out for special treatment, but this mechanism is not effective in TGAS due to the small redundancy of observations. TGAS will contain some fraction of such systems with significant deviations from the adopted five-parameter model, which will remain unrecognised in the solution. Their actual astrometric errors will be underestimated by the formal uncertainties. This may be a common characteristic of the early data releases, typically based on datasets with low redundancy and imperfect calibrations.

The impact of astrometric binaries on the derived proper motions should nevertheless be small thanks to the 24 yr baseline of TGAS. The same is true for the TGAS parallaxes as they are dominated by the [*Gaia*]{} observations and at most only a fraction of the error in the annual proper motion contributes to the parallax error. Comparing the proper motions from TGAS and 