High resolution spectroscopy for Cepheids distance determination - I. Line asymmetry N. Nardetto, D. Mourard, P. Kervella, Ph. Mathias, A. Mérand and D. Bersier A&A, 453 1 (2006) 309-319 Published online: 09 June 2006 DOI: 10.1051/0004-6361:20054333