= Feasibility of donor-wind binary contact
{title2=$\min(\zeta_0,\zeta_1)\le\zeta_*\le\max(\zeta_0,\zeta_1)$}
A <donor star> with <stellar radius response exponent> $\zeta_*$ can maintain exact contact in the <donor-wind Roche-lobe response> model for some retained fraction $0\le f\le1$ precisely when $\zeta_*$ lies between $\zeta_0$ and $\zeta_1$. This follows because the attainable <Roche-lobe radius response exponents> form that closed interval. If its endpoints coincide, all fractions work when $\zeta_*$ equals the common value and none work otherwise. With negative donor mass-loss rate, $d\log(R_d/R_L)/dt=(\zeta_*-\zeta_L)\dot M_d/M_d$ distinguishes detachment from increasing overfill. <Dynamical stability of binary mass transfer> requires the adiabatic donor response.
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