  We present an analysis of the Beam-Target double polarization asymmetry for
the photoproduction of $\Theta^+$ in $\gamma n\to \Theta^+ K^-$. We show that
this quantity can serve as a filter for the determination of the $\Theta^+$'s
spin-parity assignment near threshold. It is highly selective between $1/2^+$
and $1/2^-$ configurations due to dynamical reasons.
