Alexis Bledel, Vincent Kartheiser Engaged! Mad Men Costars to Marry

Alexis Bledel and Vincent Kartheiser arrive at the 19th Annual Screen Actors Guild Awards on January 27, 2013 in Los Angeles, California. Kevork Djansezian/Getty Images

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.

Sign up now for the Us Weekly newsletter to get breaking celebrity news, hot pics and more delivered straight to your inbox!

Want stories like these delivered straight to your phone? Download the Us Weekly iPhone app now!