Very  recently, rich systems of irregular  satellites of the giant  planets have  been  
discovered.  Their  physical and  dynamical properties provide a window on  processes 
operating in the young Solar System.   In particular  for  Uranus they  may  witness the  
mechanism leading to  the peculiar  tilt of its  rotation axis.  The  large spin obliquity  
of Uranus  (98 degrees)  is usually  attributed to  a great tangential collision (GC)  with 
another protoplanet at the  end of its formation (e.g.  Parisi \& Brunini  1997, Planet. 
Space Sci 45, 181 ).
Irregulars should have  probably existed at the moment  of the GC. The GC perturbs  the 
motion of Uranus  and allows orbits  of irregulars to change their  semimajor axis and  
eccentricity. The orbits  excited by this  perturbation  should  be  consistent with  the  
present  orbital configuration   of  the  Uranian   irregulars.   We   set  theoretical
constraints on  this scenario from  the knowledge of the  physical and dynamical properties 
of these satellites,  which also allows us to set constraints on  their capture mechanism 
(Brunini et  al.  2002, Icarus 159,  166; Parisi  et al.   2006, submitted  to Icarus;  
Maris  et al. 2001,  AJ 121, 2800;  Maris et  al.  2006,  submitted to  A$\&$A).  In order 
to test our theory, we carried out an observing programme of the Uranian  irregulars at ESO  
using  FORS2 at  VLT-UT1.   Through the satellites colors and light curves we looked for 
their shape and time dependence of  their colors and put  constraints on their origen as a
family,  their formation  (capture) mechanism,  and on  the GC  as the cause of Uranus' obliquity.