You aren't so?
Isn't it aren't I?
The first time a teacher taught me this I felt so ignorant..... so I am again...
I'm with Luis.
Just as you can say "He is, isn't he?" Or "He is, is he?", so you can say this both ways.
It should be "am I not" but it's acceptable to say "aren't I ? " to avoid sounding Shakespearian ! I would personally not use either of the suggested options.