A covariant model of the electromagnetic current for the study of   two-body scalar systems
  We present a procedure to derive a covariant electromagnetic current operator for a system made up by two scalars constituents. Using different wave functions we fitted their parameters to the experimental data of the pion form factor, obtainig great discrepancy at low momentum transfer. Introducing the Vector Meson Dominance corrective factor, we obtained a better fit to the data. 
Mario A. Acero, Maurizio De Sanctis, Carlos E. Sandoval