Two stars of masses m and 2m at a distance d rotate about their common centre of mass in free space. The period of revolution is :
${1 \over {2\pi }}\sqrt {{{{d^3}} \over {3Gm}}} $
$2\pi \sqrt {{{3Gm} \over {{d^3}}}} $
${1 \over {2\pi }}\sqrt {{{3Gm} \over {{d^3}}}} $
$2\pi \sqrt {{{{d^3}} \over {3Gm}}} $
