  The Galactic microquasar SS 433 is a member of a binary system but there is a
lack of data on the orbital velocities of the components. The emission lines of
the C II doublet at 7231 and 7236 Angstrom have been tracked nightly over two
orbital cycles. The spectra are adequate to establish that these lines are
eclipsed by the companion and hence to extract a measure of the orbital
velocity of the compact object; the lines are formed in the disk photosphere.
This velocity is 176 plus/minus 13 km/s. Could XSHOOTER do better?
