Cold CO in circumstellar disks - On the effects of photodesorption and vertical mixing F. Hersant, V. Wakelam, A. Dutrey, S. Guilloteau and E. Herbst A&A, 493 3 (2009) L49-L52 Published online: 22 December 2008 DOI: 10.1051/0004-6361:200811082