Activist investor Bill Ackman's firm Pershing Square Capital Management is boosting its stake in both Fannie Mae and Freddie Mac to more than 11 percent in each from a combined 9.8 percent in both.

Ackman's move comes a couple of weeks after the Senate Banking Committee announced an agreement on a legislation to wind down the mortgage financiers, but Ackman thinks the likelihood of the wind down is decreasing. Shares of Fannie Mae and Freddie Mac rose after the news.

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