Detection of the secondary of Algol

The first spectroscopic detection of Algol B is reported. The Na D lines of the secondary of Algol are identified on low-noise spectra, and measurements of these lines are used to determine the primary/secondary mass ratio as well as the individual masses of Algol A, B, and C. Solutions for the orbital elements yield values of 201 (+ or 6) km/s for the semiamplitude of the Algol B velocity curve and -9 (+ or - 11) km/s for the systemic velocity, in addition to a primary/secondary mass ratio of 4.6 + or - 0.1. The masses of Algol A, B, and C are estimated to be approximately 3.7, 0.81, and 1.7 solar masses, respectively. It is concluded that Algol B fills its Roche lobe and that mass transfer from Algol B to Algol A could explain the radio and X-ray emission from this eclipsing binary system.