  Ernst's solution generating technique for adding electromagnetic charge to
axisymmetric space-times in general relativity is generalised in presence of
the cosmological constant. Ernst equations for complex potentials are found and
they are traced back to an affective dual complex dynamical system, whose
symmetries are studied. In particular this method is able to generate charged,
asymptotically (A)dS black holes from their uncharged version: as an example,
it is shown explicitly how to pass from the Kerr-(A)dS to the Kerr-Newman-(A)dS
metric. A new solution describing a magnetic universe in presence of the
cosmological constant is also generated.
