  We consider a class of extremal and non-extremal four-dimensional black-hole
solutions occuring in toroidally compactified heterotic string theory, whose
ten-dimensional interpretation involves a Kaluza-Klein monopole and a
five-brane. We show that these four-dimensional solutions can be connected to
extremal and non-extremal two-dimensional heterotic black-hole solutions
through a change in the asymptotic behaviour of the harmonic functions
associated with the Kaluza-Klein monopole and with the five-brane. This change
in the asymptotic behaviour can be achieved by a sequence of S and T-S-T
duality transformations in four dimensions. These transformations are
implemented by performing a reduction on a two-torus with Lorentzian signature.
We argue that the same mechanism can be applied to extremal and non-extremal
black-hole solutions in the FHSV model.
