Schematic diagram of the sky-projected orbit of the luminous component and the Gaia epoch data. The central blue cross is the barycentre. The blue line shows the binary orbit of the luminous star. The black points are the mock Gaia epoch data, which show offsets from the orbit. These offsets are caused by the uncertainty in AL direction, indicated by the orientation of the error bar. All the points omit the proper motion and the annual motion.