A family of exact conformal field theories is constructed which describe charged black strings in three dimensions. Unlike previous charged black hole or extended black hole solutions in string theory, the low energy spacetime metric has a regular inner horizon (in addition to the event horizon) and a timelike singularity. As the charge to mass ratio approaches unity, the event horizon remains but the singularity disappears.

We present a new class of black hole solutions in Einstein-Maxwell-dilaton gravity in n ≥ 4 dimensions. These solutions have regular horizons and a singularity only at the origin. Their asymptotic behavior is neither asymptotically flat nor (anti-) de Sitter.Similar solutions exist for certain Liouville-type potentials for the dilaton.

It is shown that an arbitrarily small amount of angular momentum can qualitatively change the properties of extremal charged black holes coupled to a dilaton. In addition, the gyromagnetic ratio of these black holes is computed and an exact rotating black string solution is presented.

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It is shown that for a translationally invariant solution to string theory, spacetime duality interchanges the momentum in the symmetry direction and the axion charge per unit length. As one application, we show explicitly that charged black strings are equivalent to boosted (uncharged) black strings. The extremal black strings (which correspond to the field outside of a fundamental macroscopic string) are equivalent to plane-fronted waves describing strings moving at the speed of light. PACS numbers: 1 1.1 7.+y, 04.60.+n Despite extensive work over the past several years, we still lack a fundamental description of string theory, including a complete understanding of the basic objects that are involved and the principles which guide its construction. However, there are several hints and suggestive clues which have been uncovered. Perhaps the most important is spacetime duality [1][2][3]. This is the fact that different spacetime backgrounds correspond to equivalent solutions in string theory. Spacetime duality is usually thought of in terms of relating small distances to large distances since in the simplest example of flat space with one direction identified, the duality relates radius R to radius a'/R [4]. Although this is certainly an important aspect to duality, there are many other consequences as well. For example, it has been shown that spacetime manifolds of different topology can be equivalent as string solutions (for a recent review, see [5]). In this paper we describe another consequence of duality which concerns asymptotically defined conserved quantities. For a solution which has a translational symmetry and is asymptotically flat in the transverse directions, one can define the total energy-momentum per unit length P^ and the total charge per unit length Q associated with the three-form Hpvp. (We will refer to Q as the "axion charge/') We will show that duality interchanges Q and the component of P^ in the symmetry direction, while leaving the orthogonal components of P^ unchanged. Thus, in this sense, linear momentum is equivalent to axion charge in string theory.An immediate consequence of this equivalence is that one can add axion charge to any solution which is static as well as translationally invariant as follows: First boost the solution to obtain a nonzero linear momentum and then apply a duality transformation to convert this momentum to charge. (This procedure is related to the transformations discussed in Ref. [6].) We will illustrate this by considering the black string solutions. Black strings are one-dimensional extended objects surrounded by event horizons. Solutions to the low-energy field equations describing charged black strings in ten dimensions were found in Ref. [7]. Exact conformal field theories describing charged black strings in three dimensions were found in Ref. [8]. We will consider these three-dimensional solutions first and describe the duality between charge and momentum. For \Q\