Actress Michelle Trachtenberg, known for a bunch of TV and film roles including “Buffy the Vampire Slayer” and “Gossip Girl,” has died at the age of 39. She was found around 8 a.m. yesterday at 51-store luxury apartment complex on Central Park South. The actress recently underwent a liver transplant. Her death is not being investigated as suspicious. Trachtenberg recently posted a series of troubling pics on Instagram, in which she appeared gaunt and frail, prompting some fans to comment about her startling weight loss or ask if she was on drugs. Last January, Trachtenberg responded to fan concerns saying she was “happy and healthy” and had never had plastic surgery. Trachtenberg’s first credited role came at the age of nine on the classic 1990s Nickelodeon series “The Adventures of Pete and Pete,” on which she portrayed quirky gal-pal Nona F. Mecklenberg. She went on to star in a variety of kid-oriented movies and shows, making her film debut in 1996’s “Harriet the Spy,” in which she played the title character. As an adult, she played Dawn Summers, the sister of “Buffy’s” eponymous vampire-slaying protagonist, as well as the manipulative, calculating antagonist Georgina Sparks on “Gossip Girl,” which ran for six seasons.