SSO between two applications attached to similar Active directory

Hi,

We have two applications ABC and DEF running using different technologies. Users of both applications resides in same Active directory.
Now we want to implement SSO between these two application, i.e. when a user click on a link of website DEF in ABC website, he should not be
challenged again for username/password. My queries are:-
1) Do we need to expose ADFS service for this.
2) How this will be implemented.

Regards

TicArch