Algol is a hierarchical triple system. The inner pair has an orbital period of 2.87 days. Mass is being transferred from the sub-giant secondary to the main-sequence primary in the inner binary. Astronomers used the CHARA Array to map the orbits of the inner and outer pairs and to measure the angular sizes and mass ratios of the three components. The inner and outer orbits are nearly perpendicular.
Reference:
Imaging the Algol Triple System in the H Band with the CHARA Interferometer
Baron et al. 2012, ApJ, 752, 20