Hi. What a great forum, so many interesting things I am facing a problem with the working out for my last question in my practice paper, for my mid-year examinations. i dont know how to write the notation here, so i have just attatched a screenshot, i hope u dont mind. i just have to derive it. The hint says, use Kepler's third Law and that it is a description of the orbits of binary stars around their common centre of mass. Thank You.