1. Alty, L.J.: Kleinian signature change. Class. Quantum Gravity 11, 2523 (1994)
2. Barret, J., Gibbons, G.W., Perry, M.J., Pope, C.N., Ruback, P.: Kleinian geometry and $N=2$ superstring. Int. J. Mod. Phys. A 9, 1457 (1994)
3. Belinsky, V.A., Zakharov, V.E.: Integration of the Einstein equations by means of the inverse scattering problem technique and construction of the exact soliton solutions. Sov. Phys. JETP 75, 1953 (1978)
4. Catalano Ferraioli, D., Vinogradov, A.M.: Ricci flat $4$ -metrics with bidimensional null orbits. II. Abelian solutions. In preparation (2004)
5. Geroch, R.: A method for generating new solutions of Einstein's equation. J. Math. Phys. 12, 918 (1971)