Sharing Authentication Ticket Between ASP.NET and Sharepoint

We have ASP.NET 3.5 web application and SharePoint both are on the same web server. User authentication is done in ASP.NET application using Membership API and Forms Authentication. Now how do i share a Authentication ticket between ASP.NET application and SharePoint? Consider SharePoint is also setup for FormsAuthentication. Basically when user moves from .Net application to SharePoint we don’t want him to login again.
Im able to share authetication ticket between two ASP.Net application by having same machine key in both app's web.config file. But the same approach doesn't work between ASP.NET and SharePoint Applicattion
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