He and Si surface inhomogeneities of four Bp variable stars M. Briquet, C. Aerts, T. Lüftinger, P. De Cat, N. E. Piskunov and R. Scuflaire A&A, 413 1 (2004) 273-283 Published online: 17 December 2003 DOI: 10.1051/0004-6361:20031450