  We construct approximate kink solutions of supersymmetric open string field
theory at lowest level when non-local operators in the tachyon effective action
are fully taken into account. To this purpose we derive two duplication
formulae for products of incomplete gamma functions, which determine the level
of approximation of the solutions. The time kink is an instanton of the
Euclidean theory with a well-defined tunneling probability. The spatial kink
solution represents an unstable D9-brane decaying into a stable D8-brane. We
calculate the ratio of the brane tensions, which almost exactly reproduces the
expected value.
