HIP 98599 best fit for the 1K model. Panel a: best-fit astrometric orbit, the black cross is the system barycentre, the grey line connects it to the periapsis, and the grey dotted line joins ascending and descending nodes. Reference epochs are shown as diamonds with their position and proper motion uncertainty as hourglass-shaped shaded regions. Green to blue coloured rings show GOST data projected with the best-fit. Solid circles and slopes show the best-fit position and proper motion induced by the companion. Panel b: zoom-in for the Gaia and GOST model (grey rectangle in panel a). Panel c: residuals for the HIPPARCOS abscissa as solid grey circles, where multiple measurements per epoch have been binned and coloured by time.