Contribution of Unresolved Binary Stars to Observed Proper Motion

We consider the problem of a change of the proper motion of cluster stars if the sample contains unresolved binary stars. The paper models the distribution of the visible shift of the system brightness center (photocenter) due to the orbital motion of the binary components during 3 years (the period of the Gaia DR3 data collection). For 95 % of the systems considered, this shift is not more than 2 AU. It corresponds to a maximum addition to the tangential velocity of 0.7 km/s or an addition to the proper motions of 0.15 mas/yr at the distance of 1 kpc.