Elliott Management held about 5.7 percent stake in American Capital as of May 6, according to Thomson Reuters data.
Ares Capital shares were down 2 percent at $14.89 in early trading on Monday, while American Capital shares were up 2.4 percent at $16.00.
Wells Fargo Securities and Bank of America Merrill Lynch are Ares Capital's financial advisers for the deal. Goldman Sachs and Credit Suisse Securities LLC advised American Capital.