= Period invariant for constant-fraction donor-wind mass loss
{title2=$PM_d^{3f}M_a^3(M_d+M_a)^2=\mathrm{constant}$}
For a slowly evolving <circular orbit> with constant retained fraction $f$ and <donor-wind angular-momentum loss>, logarithmic differentiation of the <circular-binary orbital angular momentum> gives $d\log P=-3f\,d\log M_d-3\,d\log M_a-2\,d\log(M_d+M_a)$. Integration gives the displayed invariant. At $f=1$ it becomes the <period-product invariant for conservative mass transfer>, while at $f=0$ it becomes $PM^2=\mathrm{constant}$. A varying retained fraction does not give this simple power-law invariant.
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