Activist hedge fund Pershing Square Capital Management has hired private equity giant The Blackstone Group to help it fight a proposed drug store tie-up.

Pershing Square, run by William Ackman, has charged Blackstone with helping it drive up the purchase price of Longs Drug Stores Corp., which it owns an 8.8% stake in. Last week, rival CVS Caremark Corp. agreed to pay $2.6 billion for Longs, but the New York hedge fund reportedly believes that Longs’ real-estate holdings make it worth much more.

In addition to its stake in Longs, Pershing Square owns swaps that bring its total economic interest in Longs to 25.8% of shares outstanding.

According to The Wall Street Journal, a real-estate investor approached the hedge fund claiming it could make more by selling the unwanted Longs sites than CVS could, a prospect which would make Longs significantly more valuable. The investor told Pershing Square that it could try to work out a more lucrative bid for Longs from rival chain Walgreen Co.