  Certain lines in spectra of the Galactic microquasar SS 433, in particular
the brilliant H alpha line, have been interpreted as emission from a
circumbinary disk. In this interpretation the orbital speed of the glowing
material is in excess of 200 km/s and the mass of the binary system in excess
of 40 solar masses. A very simple model of excitation of disk material is in
remarkable agreement with the observations, yet it seems that the very
existence of a circumbinary disk is regarded as controversial.
  Published spectra, taken almost nightly over two orbital periods of the
binary system, show H alpha and He I lines; these were analysed as
superpositions of Gaussian components. A model in which the excitation of any
given patch of putative circumbinary material is proportional to the inverse
square of its instantaneous distance from the compact object was constructed
and compared with observations.
  The new model provides an excellent description of the observations. The
variation of the H alpha and He I spectra with orbital phase are described
quantitatively provided the radius of the emitting ring is not much greater
than the radius of the closest stable circumbinary orbit.
  The new analysis has greatly strengthened the case for a circumbinary disk
orbiting the SS 433 system with a speed of over 200 km/s and presents supposed
alternative explanations with major difficulties. If the circumbinary disk
scenario is essentially correct, the mass of the binary system must exceed 40
solar masses and the compact object must be a rather massive black hole. The
case is so strong that this possibility should be taken seriously.
