The first optical validation of an XLEO: a detection in the XMM-Newton   observation of the CDFS
  We present the first optical validation of an X-ray Line-Emitting Object (XLEO) discovered in the XMM-Newton observation of the Chandra Deep Field South. The object is an AGN at z_x ~ 0.66. An optical spectrum of the source indeed confirms and refines (z_o = 0.665) the redshift obtained from the X-ray line. The X-ray and optical properties of the source are presented and discussed. 
V. Braito, T. Maccacaro, A. Caccianiga, P. Severgnini and R. Della   Ceca