Hedge fund investor William Ackman has bought a slice of debt on Stuyvesant Town Peter/Cooper Village and could become the next owner of the sprawling Manhattan apartment complexes.

Ackman's Pershing Square Capital Management LP and Winthrop Realty Trust said on Monday they have formed a joint venture and bought the $300 million first mezzanine loan. The loan is akin to a second mortgage on the property, whose previous owners defaulted on a $3 billion senior mortgage earlier this year.

The joint venture bought the $300 million mezzanine loan for $45 million. Ackman owns 77.5 percent of the joint venture, while Winthrop owns 22.5 percent.

Following the announcement, Winthrop shares jumped 5.4 percent or 64 cents to $12.51 on the New York Stock Exchange.

An auction of the property, which includes 56 buildings and 11,227 apartments on 80 acres from 14th to 3rd Streets and bounded by First Avenue and the East River in Manhattan, has been set for August 25.

The two adjoining complexes were initially built after World War Two to house returning veterans and middle-class residents. About 25,000 people live there.

Ackman is well-versed in finding value in distressed commercial real estate. More than a year ago, he bought control of about 25 percent of General Growth Properties Inc , the No. 2 U.S. mall operator which filed for bankruptcy last year.

He has turned a roughly $25 million investment in General Growth into one that is likely to be worth well over $1 billion when the mall company emerges from bankruptcy in October, as the company expects.

Stuyvesant Town/Peter Cooper Village became a symbol of the U.S. commercial real estate bust earlier this year when Tishman Speyer, which led the group that bought the projects for $5.4 billion in 2006, defaulted on its loan payments.

Tishman Speyer had planned to renovate apartments as they became available, raising rent-stabilized units to market rents. Longer-term, they planned to sell some units or build new ones as condominiums.

But the grand plans collapsed after the market's decline and a court decision that ruled the apartments were illegally brought out of rent stabilization.

Most real estate experts have said that any new owner would likely partner with the politically well-organized tenants and convert many of the units to tenant-owned co-operative apartments.

No future owner can or will be successful without the support and participation of the tenant community, New York City Council Member Dan Garodnick, a life-long resident of the complexes who represents Stuyvesant Town and Peter Cooper Village, said in a statement on Monday.

Ackman, in a statement, said his plans include tenant ownership.

We share the Tenants' Association objective to complete a non-eviction, affordable, co-op conversion of the property, which will require the restructuring of the property's first mortgage debt, Ackman said.

Should a bidder offer $3 billion or more, the winner would be required to pay off the $300 million mezzanine loan.

However, many experts have said they believe the property today is worth far less than that amount, and unless someone steps up to bid $3 billion to cover the first mortgage, Ackman's joint venture will become the new owner and assume the mortgage.

(Reporting by Ilaina Jonas, editing by Matthew Lewis)