Yep, C#. I was considering going down to G#, but even I think that would be totally unusable. The tension at that length needs to be really high to prevent floppiness, and, as Skip will tell you, the G# is barely enough to be usable. For the C#, I think even that is borderline at best. And don't plan on playing indoors, because it makes everything in the room resonate. You'll think you have a bad speaker or something, because the whole house starts rattling.