Blake Lively and Alyson Hannigan are among the former "Gossip Girl" and "Buffy" actors mourning the death of their one-time co-star, Michelle Trachtenberg.

Actor Sarah Michelle Gellar paid tribute to her “Buffy the Vampire Slayer” costar Michelle Trachtenberg on Thursday, a day after police foung the 39-year-old dead in her New York City home.

Michelle Trachtenberg has died at the age of 39. Sources say the actress was found by her mother around 8 a.m. on Wednesday at One Columbus Place, a high-end apartment complex in Manhattan. Insiders revealed that Trachtenberg had undergone a liver transplant in recent months.