Signature change induces compactification.

It is shown – using a FRW model with S 3 × S 6 as spatial sections and a positive cosmological constant – that classical signature change implies a new compactification mechanism. The internal scale factor is of the order Λ −1/2 , and the solutions are stable against small perturbatons. In the case of compactified S 6 , it is shown that the effective four-dimensional space-time metric has Lorentzian signature, undergoes exponential inflation in S 3 and is unique. Speculations concerning relations to quantum cosmology and conceivable modifications are added.