If a binary system is close enough to us, we can see each component as a separate point of light, each of which moves round the other in a regular manner. At each point in time we can plot the positions of each star in the sky. In time each star traces out a spiral projected on to the plane of the sky If we remove the motion along the spiral from our observations, we find that each star traces out a similar ellipse with apparent eccentricity e*, say. It is possible to find the orbital elements, We can also measure the ratio of the angular distances of each star from the centre of mass (r2r1) and this is the MASS RATIO q = m1/m2. This is as much as we can deduce without additional information: for example, we only know the apparent angular size of the system and do not know the physical size. However, if we have an indepen¬dent estimate of the distance to the system, for example by parallax, we can deduce the physical size. Hence from Kepler’s law we can deduce the total mass of the system, and knowing q, the mass of each star.

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