single sign on among multiple instances of classified starter kit

I am using classified starter kit for various purposes like for business listing,classified listing and event listing.These are 3 independent application with their membership information stored in their independent database.I have used this 3 listings in a portal .I need a single sign on now.
I tried to do with machinekey & cookie .This way user able to login but as there is no record in that database for that user.So even if he post  anything in that application it stores memberid field as zero. So in this case what should be done. i am stuck here since few weeks. please reply to me as soon as possible.
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