The Nappi-Witten example and gravitational waves
  The vanishing of the anomaly in the recent example of Nappi and Witten, constructed from the Wess-Zumino-Witten model based on a certain non-semisimple group, follows from a more general result valid for gravitational waves. The construction of the metric is explained.} 
C. Duval, Z. Horv\'ath, P.A. Horv\'athy