Interferometric Observations of Algol

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We determined the spatial orientation of the Algol AB close pair orbital plane using optical interferometry with the CHARA Array, and radio interferometry with the European VLBI Network (EVN). We found the longitude of the line of nodes for the close pair being Ω₁=48°±2° and the mutual inclination of the orbital planes of the close and the wide pairs being 95°±3°. This latter value differs by 5° from the formerly known 100° which would imply a very fast inclination variation of the system, not supported by the photometric observations. We also investigated the dynamics of the system with numerical integration of the equations of motions using our result as an initial condition. We found large variations in the inclination of the close pair (its amplitude 170°) with a period of about 20 millenia. This result is in good agreement with the photometrically observed change of amplitude in Algol's primary minimum.