Numerical test for the linear approximations (solid black lines), given in Eqs. (8) and (14). Top: Expected visibility amplitudes V_s+c and closure phases Φ_s+c of a binary system composed of a central star and a companion (dashed lines), exemplarily modeled for κ Tuc A and a faint companion at the projected distance of ≈ 1.4 au, assuming a companion-to-star flux ratio of f=1% (for details refer to Fig. 1). Bottom: Residuals (res) between the exact model and the approximation, shown for the visibility and closure phase.