Non-parallel Electric and Magnectic Fields in a Gravitational   Background, Stationary Gravitational Waves and Gravitons
  The existence of an electromagnectic field with parallel electric and magnetic components is readdressed in the presence of a gravitational field. A non-parallel solution is shown to exist. Next, we analyse the possibility of finding stationary gravitational waves in de nature. Finaly, We construct a D=4 effective quantum gravity model. Tree-level unitarity is verified. 
Carlos Pinheiro (Dep. Fisica-UFES), J.A. Helayel-Neto (CBPF) and   Gilmar S. Dias (UFES and ETFES)