1. For a recent review, see: I. Bars, “Curved Space-time Geometry for Strings ...”, hep-th 930942, in Perspectives in Mathematical physics” Vol.III, Eds. R. Penner and S.T.Yau, International Press (1994), page 51.
2. A.A. Tseytlin, reviews, hep-th/9408040, 9410008.
3. G.T.Horowitz and A.A. Tseytlin, hep-th/9409021.
4. C. Nappi and E. Witten, hep-th/9310112.
5. K. Sfetsos, hep-th/9311093, K. Sfetsos and A.A. Tseytlin hep-th/9404063.