ABOUT A CLASS OF EXACT STRING BACKGROUNDS

We investigate a class of string backgrounds which have one conserved chiral null current on the world sheet. In the target space they have a null Killing vector and unbroken supersymmetries. This class also known as the chiral null model is a generalization of the gravitational wove and fundamental string background and is exact in the alpha' 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 = O and confirm the H-monopole as an N-L = 1 string state.