Integrable models based on non-semi-simple groups and plane wave target spacetimes

We initiate the construction of integrable $\lambda$-deformed WZW models based on non-semisimple groups. We focus on the four-dimensional case whose underlying symmetries are based on the non-semisimple group $E_2^c$. The corresponding gravitational backgrounds of Lorentzian signature are plane waves which can be obtained as Penrose limits of the $\lambda$-deformed $SU(2)$ background times a timelike coordinate for appropriate choices of the $\lambda$-matrix. We construct two such deformations which we demonstrate to be integrable. They both deform the Nappi-Witten plane wave and are inequivalent. Nevertheless, they have the same underlying symmetry algebra which is a Saletan-type contraction of that for the $\lambda$-deformed $SU(2)$ background with a timelike direction. We also construct a plane wave from the Penrose limit of the $\lambda$-deformation of the $\nicefrac{SU(2)}{U(1)}$ coset CFT times a timelike coordinate which represents the deformation of a logarithmic CFT constructed in the past. Finally, we briefly consider contractions based on the simplest Yang-baxter $\sigma$-models.