About a class of exact string backgrounds

TL;DR

The paper studies exact string backgrounds with a null Killing vector, introducing the chiral null model that unifies gravitational wave and fundamental string backgrounds and remains exact in $\alpha'$. By dimensional reduction from 10D to 4D, it yields an Israel-Wilson-Perjés type metric (IWP) coupled to multiple gauge fields and scalars, fully specified by eight harmonic functions. Special choices reproduce Taub-NUT and rotating black holes, revealing T-self-dual points and massless limits, while S-duality generates electric-magnetic families including H-monopoles; the analysis links these geometries to elementary string states via oscillator levels. The results highlight a robust duality web that preserves exactness under transformations and offer a string-state interpretation with $N_R=1/2$, providing insight into the nonperturbative structure of exact string backgrounds and their physical relevance.

Abstract

This model also known as chiral null model is a generalization of the gravitational wave and fundamental string background and is exact in the $\a'$ expansion. The reduction to 4 dimensions yields a stationary IWP solution which couples to 7 gauge fields (one gravi-photon and 6 matter gauge fields) and 4 scalars. Special cases are the Taub-NUT geometry and rotating black holes. These solutions possess a T-self-dual point where the black hole becomes massless. Discussing the S-duality we show that the Taub-NUT geometry allows an S-self-dual point and that the electric black hole corresponds to a magnetic black hole or an H-monopole. We could identify the massless black hole as $N_L=0$ and confirm the H-monopole as an $N_L=1$ string states.