Fundamental planes for the definition of the reference angles. The angles in black are determined directly from the observations (Table 1), while the two angles in blue (I₁,ΔΩ) are the missing angles in the observations that prevent the full characterization of the orbits in the system. The angles in red are also undetermined, but related with the other angles. They are linked to the physical system, are therefore independent of the reference frame, and more suitable to study the dynamics.