The mass of the classical Cepheid SU Cygni

Velocities of the companion of the classical Cepheid SU Cyg have been measured from 13 IUE high-dispersion spectra. Because the companion is itself a member of a short-period binary system, its short-period orbit and the orbital amplitude of the center of mass in the long-period system are determined from a triple system solution. The velocity amplitude of the short-period center of mass in the long-period orbit is 32.2 + or - 1.6 km/s. This leads to a mass ratio between the mass of the Cepheid and the sum of the masses of the companions of 1.07 + or - 0.05. Combining this mass ratio with the mass function from the Cepheid orbit produces a purely dynamical lower limit to the mass of the Cepheid of 5.9 + or - 0.4 solar masses, in good agreement with the evolutionary mass for the Cepheid without mass loss or convective overshoot near the main sequence.