[AUCTeX] Latex counters

[Christophe FARGES]

Hello,

I've got a question about latex (not auctex). I'm sorry to post here but 
I need an answer very soon and I know that this mailing list usually 
answer fast. Here is my question: is it possible to share a counter 
between 2 or more latex environments, and how can I do so? More 
precisely, I'm finishing writing my thesis, and I've got those environments:

\newtheorem{theoreme}{Th\'eor\`eme}[chapter]
\newtheorem{corollaire}{Corollaire}[chapter]
\newtheorem{lemme}{Lemme}[chapter]

So when I create the first theorem, I've got:
Theorem I.1
After that, if I create the first lemma , I've got:
Lemma I.1

But I would like to have: Lemma I.2 ....

Thanks in advance for your help,
Christophe

Remark: I'd like to keep the reinitialization of the counter at the 
beginning of each chapter as it works right now (for example: first 
theorem of chapter II is: Theorem II.1)