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Most binaries are undetected. Astrometric reductions of a system using theassumption that the object moves like a single point mass can be biased byunresolved binary stars. The discrepancy between the centre of mass of thesystem (which moves like a point mass) and the centre of light (which is whatwe observe) introduces additional motion. We explore the extent to which binarysystems affect single object models fit to astrometric data. This tells us howobservations are diluted by binaries and which systems cause the largestdiscrepancies - but also allows us to make inferences about the binarity ofpopulations based on observed astrometric error. By examining a sample of mockobservations, we show that binaries with periods close to one year can mimicparallax and thus bias distance measurements, whilst long period binaries canintroduce significant apparent proper motion. Whilst these changes can soak upsome of the error introduced by the binary, the total deviation from the bestfitting model can be translated into a lower limit on the on-sky separation ofthe pair. Throughout we link these predictions to data from the Gaia satellite,whilst leaving the conclusions generalizable to other surveys.

Binary deviations from single object astrometry