X-ray spectral variability in PG 1535+547: the changing look of a “soft X-ray weak” AGN L. Ballo, M. Giustini, N. Schartel, M. Cappi, E. Jiménez-Bailón, E. Piconcelli, M. Santos-Lleó and C. Vignali A&A, 483 1 (2008) 137-149 Published online: 04 March 2008 DOI: 10.1051/0004-6361:20079075