String backgrounds and LCFT

We describe a large class of exact string backgrounds with a null Killing vector arising, via a limiting à la Penrose procedure, from string backgrounds corresponding to coset conformal field theories for compact groups GN/HN times a free time-like boson U(1)−N . In this way a class of novel logarithmic conformal field theories (LCFT) emerges, that includes the one constructed recently as an N → ∞ limit of the SU(2)N/U(1)× U(1)−N theory. We explicitly give the exact operator algebra for the basic chiral fields as well as their representation in terms of free bosons, even though these are not known in general at finite N . We also compute four-point functions of various operators in the theory. For the cases of the fourand five-dimensional models, corresponding to a limit of the theory SO(D+1)N/SO(D)N ×U(1)−N for D = 3 and 4, we also present the explicit expressions for the background fields.