Billionaire Steven A. Cohen’s hedge fund is up about 10% this year. But a significant part of that return has come from a money manager who doesn’t work at Cohen’s fund.

Cohen is widely viewed on Wall Street as one of the best traders of all time. From 1993 through 2011, his firm only trailed the S&P 500 once, according to a Wall Street Journal calculation. Cohen’s former firm, SAC Capital Advisors, used to make more than 60% in some years, a client document shows.