Multiple shock waves in the atmosphere of the Cepheid X Sagittarii? P. Mathias, D. Gillet, A. B. Fokin, N. Nardetto, P. Kervella and D. Mourard A&A, 457 2 (2006) 575-579 Published online: 12 September 2006 DOI: 10.1051/0004-6361:20065299