Is there a single sign-on add-on included if we purchase a professional license?

Yes, each Professional Edition license includes a single sign-on authentication modules.

These modules include:

• Windows Active Directory - for single sign-on integration with AD authentication (common for intranets and extranets).
• ASP.NET Authentication - for single sign-on integration with an existing ASP.NET web application.
• Custom Cookie - for single sign-on for just about any other type of web application.

The Professional Edition additionally includes an Enterprise Search license as well.