On the eta-invariant in the 4D chiral U(1) theory
  In contrast to its counterpart in a vector theory, the effective action of a chiral gauge theory may have a non-vanishing imaginary part. It consists of the so-called Chern-Simons form, which encodes the anomaly and a gauge invariant piece related to the eta-invariant of some extended Dirac operator. We investigate the imaginary part of the effective action in the 4D chiral U(1) theory within the continuum fermion approach. Perturbative analysis indicates in general a non-vanishing imaginary part of the effective action. However, after anomaly cancellation we find only a small value of the imaginary part for the investigated configurations. 
V. Bornyakov, A. Hoferichter, G. Schierholz, A. Thimm