= Nova-induced binary detachment
An idealized <classical nova> that slowly ejects <mass> $\delta m$ isotropically from the accretor, compared with the <orbital period>, but quickly compared with the mass-transfer timescale, expands a circular <binary star>'s <Roche lobe>. The ejecta carry accretor <specific angular momentum> $j_1/J=M_2/(M_1M)=q/M$. Taking $\delta J/J=-q\delta m/M$, $\delta M_1=-\delta m$ and $\delta M_2=0$ in $J=M_1M_2\sqrt{Ga/M}$ proves $\delta a/a=\delta m/M$. The lobe law $R_L\propto a(M_2/M)^{1/3}$ then gives $\delta R_L/R_L=4\delta m/(3M)$. A donor whose radius is unchanged moves inside its lobe.
For constant external loss $\Gamma=-\dot J>0$ and a donor sequence $R_2\propto M_2^\zeta$, the fractional gap closes at rate $2\Gamma/J$, giving $t_d=2J\delta m/(3M\Gamma)$. The <binary mass-transfer contact equation> gives accumulation time $t_s=J\delta m(\zeta+5/3-2q)/(2M_2\Gamma)$. Therefore
$$
\frac{t_d}{t_s}=\frac{4q}{(1+q)(3\zeta+5-6q)}.
$$
This first-order result requires $\delta m/M\ll1$, a positive contact denominator and a fixed donor radius during the eruption. At $\zeta=1$ it reduces to $2q/[(1+q)(4-3q)]$.
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