[ESS] trouble with accented character in the prompt windows when I evaluate a command

[LaurentEss]

Dear ESS-Help-list,

       I use .Rnw file and polymode. In a code R chunk, when I evaluate 
for example the line cat("test équation") that is to say with an 
accented character "é" I have the following error message : Error: 
invalid multiple character in parser

In the emacs windows which embodies the R session, I typed: C-h v 
buffer-file-coding-system RET and I can read in the help windows: Its 
value is 'utf-8-unix'. When I compile the .Rnw and then create the pdf 
file there is no trouble the accented characters are taken into account 
and my document is correct.

But it is annoying to debug my R chunks, I have to remove the accented 
characters to rewrite them after in the final version.

How could I resolve this trouble ?

Thank you

Laurent