Three Citadel veterans, who helped run the Chicago hedge fund firm's profitable stock- and options-trading unit before leaving more than a year ago, have reunited to start a computer-driven trading business.

Matthew Andresen, former co-head of the Citadel group that makes markets in stocks and equity derivatives, and Jason Lehman, who ran the global options business, are co-chief executives of newly formed Headlands Technologies, they say.