Ground-based detection of sodium in the transmission spectrum of exoplanet HD 209458b I. A. G. Snellen, S. Albrecht, E. J. W. de Mooij and R. S. Le Poole A&A, 487 1 (2008) 357-362 Published online: 15 May 2008 DOI: 10.1051/0004-6361:200809762