Understanding the on-off states in CAL 83

CAL 83 is the canonical closebinary supersoft Xray source and also among thetwo rare systems to show repeated on/off Xray states. These systematic changesare anticorrelated with the optical brightness and closely connected toprocesses occuring at near Eddington accretion rates. We propose an opticallytriggered XMM monitoring of CAL 83 through one Xray bright state following itsevolution with many short pointings (5 ksec each). This will provide new insightinto the yet unknown physical mechanism behind the Xray on/off states bytesting three alternatives: (a) Luminosity changes at constant temperature thatindicate a cessation of the shell burning; (b) Temperature changes due tovariations of the effective radius of the white dwarf; (c) Absorption, e.g. by a variable wind. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]