I tried the following:
a) Sun (negative mass) and earth (positive mass). Earth gets repelled.
b) Sun (positive mass)and earth (negative mass). Earth orbits sun. c) This is just for fun: Throw a few Ceres type planets on a negative earth. The effect is amazing:
There is endless debate on the internet about (b). With some simple google searching you are about to find a ton of theorycraft, from so called scientists, space geeks, wikipedia, etc. Read them, they are fun, but be careful what you take for granted.
Anyways just wanted to share this simple idea so more people can try it and enjoy themselves.