"The pair clearly have history going back a long way, so it looks like a rather lovely case of good friends becoming partners," contactmusic.com quoted a source as saying.
"Kartheiser proposed a few weeks ago. She's been wearing her ring. It's huge," the source added.
Kartheiser has previously dated actress Rachael Leigh Cook, whereas Bledel dated Milo Ventimiglia, who she was with for more than three years.