Force authentication

I use a plugin externa DB authentication that works fine allowing me to match auth users against an external DB. But I need a further step.

I get users already logged in on another site, for them i have they login and password, if I use them to login on WP using the plugin above it works like a charm, but i don’t want to have user to login on WP but getting they login data and send it in some way to WP to make them being logged in a full transparent way but passing trhought the ext db auth plugin. In other words i wanna pass usersname e password not to get the auth cookies without having to make people login or redirecting to WP login URL with a redirect… to not have the page reloaded.

Any idea how to? I tried to call wp_authenticate passing users and password but no luck it loops somewhere.

It’s a bit urgent… thanks in advance.