Why Is 2r Used in Calculating the Masses in Plaskett's Binary System?

[unknown_2]

hey
i've been having trouble understanding a part in this question. i'll just map out the basics to the question: 2 stars with a velocity of 220km/s are orbiting at equal distance. the masses of the stars are equal. period = 14.4 days it asks to find the mass of each star.

i understand that u can use kepler's law to get the answer.

GMm/r² = mv²/r

-which is what i used to come up with my answer seeing that i found the mass relative to a center mass. but according to the solution it's suppose to be:

GMm/(2r)² = mv²/r

my question is, why is r denoted 2r in order to arrive at a correct answer?

 

[Redbelly98]

On the left hand side, you use the distance between the 2 stars.

On the right hand side, you use the distance from a star to the center of the circular orbit.