The two actors previously starred opposite each other in AMC drama series Mad Men.
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Bledel guest starred as Beth Dawes, who entered a love affair with Kartheiser's Pete Campbell in the latest season.
A source told Us Weekly: "They have been dating for a few months."
Kartheiser apparently attended the play Regrets, starring Bledel, in New York back in March.
The source added: "Vincent is smart and charming. He's nothing like his character!"
Mad Men airs on AMC in the US and Sky Atlantic in the UK.
Watch a trailer for Mad Men below:
