Hi,
Thanks a lot for your answer .
I took a look at lib-auth.php, and found printLoginForm function,and then at comment_form folder . I think it's gonna be very ,very complex to add this option on the login form ... :-( . But thanks a lot for your answer.
And you're totally right it's not usefull but my boss asked , he's not a developer and even doesn't know what is for a captcha, but he wants his captcha . You're right but he's the boss and i don't have many choices.
Thanks again