= Gas drag on a planetesimal binary
For linear <gas drag> with <aerodynamic stopping times> $t_1,t_2$, masses $m_1,m_2$, total mass $m=m_1+m_2$, and relative velocity $\mathbf v$, define
$$
\kappa=\frac{m_1/t_2+m_2/t_1}{m},\qquad
\kappa_b=\frac{m_1/t_1+m_2/t_2}{m},\qquad
\Delta=\frac1{t_2}-\frac1{t_1}.
$$
If $\mathbf w$ is the gas velocity relative to the <centre of mass>, the relative drag acceleration is $-\kappa\mathbf v+\Delta\mathbf w$. The first term dissipates internal <specific orbital energy>; the second is a differential headwind that vanishes for equal <aerodynamic stopping times>.
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