The pair met during filming for the fifth season of the hit TV drama, in which the Gilmore Girls actress plays the sex-starved mistress of Kartheiser's married character Pete Campbell.
Their chemistry onscreen spilled over into real life and they made their first red carpet appearance as a couple on Wednesday (10Oct12) at the Macallan and Annie Leibovitz Photo Launch in New York.
Kartheiser admits their conflicting schedules make it difficult to maintain a healthy romance, so they have imposed a relationship rule and vowed to make an extra special effort to reunite at least once every few days - no matter where they are in the world.
He tells UsMagazine.com, ""I'm a really lucky guy. The most important thing, for us, is to make sure that we see each other in the flesh at least once every 10 days.
""All the other stuff is good, like FaceTime (iPhone video calling) and the texting... I love seeing her. You gotta make the flight!""