Combined vector point diagram of all proper motions, both relative and absolute, relevant for the present study. Horizontal axis is μ_α cos δ and vertical axis is μ_δ in ICRS orientation. The labelled red triangles denote the absolute measurements of the proper motion of Albireo A by HIPPARCOS (epoch J1991.25), by Gaia DR2 (epoch J2015.5), and by the difference between the Gaia DR2 and HIPPARCOS positions. The open black squares denote the relative motion of Aa as derived from the Scardia et al. (2008) orbit solution for the pair Aa,Ac (i.e. from a mirror image of Fig. 1) for the same epochs. The blue circles show the same data, but shifted by an assumed systemic motion of Aa,Ac of (+7.0,+4.6) mas year⁻¹ to give a best-possible fit to the absolute motions of A (red triangles). That absolute systemic motion is indicated by the green diamond, to be compared with the two violet circles showing the absolute proper motions of B measured by HIPPARCOS (lower left) and Gaia DR2 (upper right). Their error bars are about the size of the symbols.