SS 433: Two robust determinations fix the mass ratio

Context. The unique Galactic microquasar SS 433 is an X-ray binary with a 13.08 day orbital period. Over some forty years, estimates of the mass of the compact object have spanned the range 1-30 solar masses; from neutron star to massive stellar black hole. Aims. To fix the mass ratio q and hence the mass of the super-Eddington accretor. Methods. We present a new and robust estimate of q derived from data on the circumbinary disk and compare it with a recent determination from the observed constancy of the binary period. Results. These two robust analyses agree to a remarkable extent. They reinforce each other; q is similar to 0.7 and the mass of the compact object 15 +/- 2 M-circle dot. Conclusions. The mass is such as to identify the compact object as a massive stellar black hole.