External magnetic fields in gauge theories
  A general discussion is presented of the response of a gauge-field system to external magnetic fields, in the light of a theorem due to S. Elitzur. As a result a natural understanding emerges of some recent puzzling results from lattice MC simulations, as well as of the phenomenon of ``perfect diamagnetism'' of non-abelian gauge theories, discovered almost ten years ago. 
L. Gamberale, G. Preparata and She-Sheng XUE