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I propose ISOCAM observations of the recently identified and truly unique QSO
1556+3517 (z = 1.48). Indeed, 1556+3517 is to date the only broad absorption
line (BAL) QSO which is also a genuine and powerful radio source. Other
peculiarities of 1556+3517 are (1) it belongs to the relatively rare sub-class
of low ionisation BAL QSOs (2) it shows absorption lines from metastable
levels of iron (3) its optical spectrum is very red. All these properties
suggest that 1556+3517 is a proto-QSO caught in a very early stage of evolution
when the source is still shrouded by a thick and hardly ionized cocoon of
dust and gas. Moreover, by being the only source which combines the mutually
exclusive properties of radio-loudness and BAL, 1556+3517 may yield clues as
to the origin of both phenomena. The ISOCAM observations are aimed at
measuring the thickness of the dust cocoon and studying the environment of
the nuclear source, searching in particular for signs of galactic scale burst
of star formation, cooling flows or merging, which may all be related to the
onset of nuclear activity.