lcd luminosity keys disabled

Hello everybody,
For a couple of weeks (probably after an update as I'm using testing) I cannot setup the luminosity of my HP laptop's screen : the dedicated key combinations ("Fn" + F7 and "Fn" + F8) don't do their job anymore. I suspect they are intercepted somehow by a program (I'm using gnome 2.18 and xorg 7.3) but my knowledge of X key binding is very poor. I didn't find a similar problem with google. Still I particulary suspect gnome because it can usefully intercept for example "Fn" + F3 (= "www" key recognize by gnome as "0xb2") to launch iceweasel. Does anyone have some clue to enable these keys once again ?

Thank you for your answers and please forget my english mistakes.

                Pierre H.

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