Using gravitational force , the distance between the star is R. If you apply this force as centripetal force , the radius of the circular orbit is only R/2. A twin star circulate about each other as both are just as massive. The trap is here where most students think that the distance between the star is the radius of the circle . Here is not the same as Earth and Moon where the Moon circle around the Earth and the distance between them R is the radius of the circle,R.
