Vincent Kartheiser

Their Mad Men characters had an ill-fated affair, but off-camera, Alexis Bledel and Vincent Kartheiser are excitedly celebrating their engagement, Bledel’s rep confirms exclusively to Us Weekly. "They couldn’t be happier," says a pal of the Gilmore Girls alum, 31, and Kartheiser, 33.

Last June, Us broke news that the twosome — Bledel played the mentally imbalanced mistress to his character, ad exec Pete Campbell — had been "dating for a few months."

Says another friend: "Vincent proposed a few weeks ago. She’s been wearing her ring. It’s huge!" In the past, Kartheiser has been linked to actress Rachael Leigh Cook; Bledel dated her Gilmore Girls costar (and on-screen love interest) Milo Ventimiglia for more than three years.

"They are so cute together!" another source tells Us of the spouses-to-be. It will be the first marriage for both, who attended the October nuptials of Bledel’s Sisterhood of the Traveling Pants costar Amber Tamblyn and David Cross.