Department of Physics
Ecole Normale Superieure
24 rue Lhomond
75005 Paris France

Keir Neuman

National Institute of Health
50 South Dr. Building 50,
3517 MSC-8013
Bethesda MD 20892-8013 USA.

Terence Strick

Institut Jacques Monod
University of Paris – Diderot
4 rue M.-A. Lagroua Weill-Halle
75205 Paris Cedex 13 France

strick@ijm.jussieu.fr