Sharing authentication between 3 websites

Sharing authentication between 3 websites

I have a main website, with 2 sub-sites. I want to create a simple way to
authenticate on a website such that I can view the other 2 sub-sites.
THe websites are setupa as follows:
www.example.com/
www.example.com/subsite1
www.example.com/subsite2
As a further requirement, subsite1 and subsite2 may be re-used on a
different website.
Could I use the built in Authentication module in ASP.NET for this? Or do
I have to create my own custom cookie?