[TYPO3] Login system and subdomains

[Cray Flatline]

I have main site and two subdomains

domain.com
chat.domain.com
forum.domain.com

And when I logging in on the domain.com, on the chat.domain.com - I 
still don't logged in. If I login on the chat.domain.com - I don't 
logged in on the other domains.

Does anybody help me to solve this thing? I want to make something, so 
if user logged in on the main domain, he becomes automaticaly logged in 
on the subdomains? Any propositions?

Thanks a lot...


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W BR, Cray Flatline