if a spacecraft is set into orbit around a binary star system such that it is collinear with the two stars. if the distance of the spacecraft from each of the stars is $R$ and $r$ then what will be the ratio of the masses of the stars? is this a stable one?
In general this situation expressed in the title is true of arrangements with a light body in the first, second or third Lagrange point associated with a pair of orbiting massive bodies.
Such arrangements are, however, unstable and won't persist in real life without some kind of station-keeping.