Appearance of Boulware-Deser ghost in bigravity with doubly coupled   matter
  We discuss the ghost freeness in the case when we add matter coupled to two metrics to the ghost-free bigravity. In this paper we show that the Boulware-Deser ghost generally revives in the presence of doubly coupled matter and that ghost freeness strongly restricts the model of kinetically doubly coupled matter. This result may anticipate difficulties in the attempt to derive the ghost-free bigravity as a low-energy effective theory, starting with a model applicable at high energies. 
Yasuho Yamashita, Antonio De Felice, Takahiro Tanaka
hep-th gr-qc