= Laboratory paths of an equal-mass binary
For equal masses and relative <position> $\mathbf r$, the individual positions are $\mathbf r_A=\mathbf R-\mathbf r/2$ and $\mathbf r_B=\mathbf R+\mathbf r/2$, where the <center of mass> has uniform <velocity> in an isolated system. Bound <inverse-square force> motion makes $\mathbf r$ an <ellipse>; each mass follows a half-size ellipse in the center-of-mass frame. Adding a nonzero center-of-mass drift generally makes their laboratory <trajectories> nonclosed. The relative orbital period is not necessarily a period of either laboratory position.
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