  The problem of estimating the total mass of a visual binary when its orbit is
incomplete is treated with Bayesian methods. The posterior mean of a mass
estimator is approximated by a triple integral over orbital period, time of
periastron and orbital eccentricity. This reduction to 3-D from the 7-D space
defined by the conventional Campbell parameters is achieved by adopting the
Thiele-Innes elements and exploiting the linearity with respect to the four
Thiele-Innes constants. The formalism is tested on synthetic observational data
covering a variable fraction of a model binary's orbit. The posterior mean of
the mass estimator is numerically found to be unbiased when the data cover >
40% of the orbit.
