  We prove the sharp domain of dependence property for solutions to subelliptic
wave equations for sums of squares of vector fields satisfying H\"ormander
bracket condition. We deduce a unique continuation property for the square root
of subelliptic Laplace operators under an additional analyticity condition.
Then, with a different, more involved method, we prove the same result of
unique continuation for more general $s$-powers ($0<s<1$).
