Astrometric measurements of stars A (squares) and B (diamonds) relatively to C. Measurements are shifted so that the one in Kcont from 2016 October is on (0, 0). Each measurement (date, filter) is denoted with a different colour. The grey line shows the apparent motion on the sky over the course of the observations, expected from the parallax and proper motion values from Gaia DR1, and if the star C was a steady background object. Black circles mark the moments of archival observations (2013 June and 2014 July).