Interpolating black holes in dilaton - axion gravity

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Abstract
A 2p + 5 parametric family of black holes is constructed in dilaton - axion gravity with p vector fields using a holomorphic representation of U-duality in three dimensions. The metric of the non-extremal black holes has a Reissner - Nordström-type structure and generically possesses an internal horizon. However, in the extremal limit the generic solution exhibits a dilatonic-type null singularity. The solution thus interpolates between the well known Reissner - Nordström and charged dilatonic spacetimes. Only in the case of the orthogonal electric and magnetic charges (if p > 1) may the extremal solution have a non-singular event horizon.