  We consider the helical reduction of the wave equation with an arbitrary
source on $(n+1)$-dimensional Minkowski space, $n\geq2$. The reduced equation
is of mixed elliptic-hyperbolic type on ${\bf R}^n$. We obtain a uniqueness
theorem for solutions on a domain consisting of an $n$-dimensional ball $B$
centered on the reduction of the axis of helical symmetry and satisfying
ingoing or outgoing Sommerfeld conditions on $\partial B\approx S^{n-1}$.
Non-linear generalizations of such boundary value problems (with $n=3$) arise
in the intermediate phase of binary inspiral in general relativity.
