Non-Universal Anomalies in Heterotic String Constructions
  We investigate anomalies on heterotic orbifolds and their blowups. We give a simple example of an orbifold blowup which contains anomalous U(1) symmetries that are canceled by axions which couple non-universally to the different gauge groups, thus clarifying some confusion which recently arose in the literature concerning anomaly universality. We argue that non-universal axionic couplings are the general case, and that the couplings are only universal in the case of orbifolds. We comment on the consequences of this non-universality for discrete R symmetries. We furthermore investigate the origin of discrete (R and non-R) symmetries on smooth Calabi-Yau manifolds. 
Christoph L\"udeling, Fabian Ruehle, Clemens Wieck
hep-th