1. The solution is given by the Kepler's Law Calculator. Alternatively, from Kepler's modified 3rd Law we have
where P is the period in years, a is the average separation in AU, and M 1 and M 2 are the masses of the stars. Thus,
2. Observation only gives us the angular separation for the components of the binary. We need the distance and simple trigonometry to convert that to a true distance (e.g., in astronomical units) so that we can use it in Kepler's 3rd law to determine masses.
3. Better observing techniques may allow the formerly unseen companion to become visible, as happened in the case of Sirius.