SSO on web service

Hi all,

I have this requirement:

1. Retrieve information from some tables in ERP2005 system.

2. Call this web service from external system such as WebLogic.

3. SSO should be enabled i.e. the user does not need to enter id and password again when consuming web service.

What is the best approach to do this scenario? Any document on how to do this?

Thanks,

YJ.