R help - Jan 2008 - autocorrelation by group in mixed model

Hi all!
 
(How) is it possible to fit a mixed model with group specific auto-correlation
structure ? For instance, not all my groups display auto-correlation so I would
like to use a corMatrix (corAR1) only for the auto-correlated ones. If I
construct manually a
the corMatrix, is it possible to use it  as input somehow?
 
thank you!
 
Irene