>>1178958You might not even need a shelter. There are so many huts/shelters on the AT that you can just sleep in those on rainy nights, and sleep in your bivy on dry ones. That said, I'd always carry a light tarp just to be sure. You don't want to get caught off-guard by a rainstorm without any shelter. If you're american you can get the Borah tarp. It's cheap, light but pretty small (narrow). I wouldn't use it as a main rain shelter but it's fine as a backup on the AT.
http://www.borahgear.com/tarps.html 