1. Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma , 440 W. Brooks Street, Norman, OK 73019, USA
2. Département de Physique, Université de Montréal , C.P. 6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7, Canada
3. Smithsonian Astrophysical Observatory , 60 Garden Street, Cambridge, MA 02138, USA