Add the remaining astrometric orbital elements to the fit parameters, and
vary the secondary mass only. If you have a measurement of the parallax, say
from the Hipparcos catalogue, you can add this measurement using the
`set_parallax` procedure. In this case, you may add also the primary
mass to the fit parameters as the system is now constrained just like
a double-lined spectroscopic binary.

Christian Hummel 2015-04-28