Multi stores one login on different domains..

I have 5 domains and in every one have oscommerce installed on it, they
share one database all togother and everyone have his products, the
customers are in the all five systems same, if someone registersw on
one system he is registered on thers, but the problem I have now that
if a user logins on one and jumps to another he must login again, how
can I make it as a global login on the 5 oscommerces, so the user don't
have to login every time again..
Thank you for your help in advance..