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Thirty-eight US billionaires have pledged at least 50% of their wealth to charity through a campaign started by investor Warren Buffett and Microsoft founder Bill Gates. Here is a look at some of those who have signed up to project.

Bill Gates

According to Forbes Magazine's Billionaires List, the Microsoft mogul is currently worth $53bn (£33bn, 40bn euros), although this enormous figure still only puts him in second place on the world's rich list behind Mexican telecoms billionaire Carlos Slim Helu. Despite having left Microsoft he is still the company's largest single stockholder. In recent years he has focussed on philanthropy, donating large amounts of money to various causes through the Bill & Melinda Gates Foundation, established in 2000.

Warren Buffett

One of the most successful investors in the world, and frequently referred to as "legendary investor Warren Buffett". In 2009 he was listed as the world's richest person. However, following the global economic crisis he now has to settle for third place and a fortune worth $47bn, according to Forbes. Mr Buffett is also widely known for his philanthropic work having promised to give away 99% of his fortune to charitable causes, primarily via the Gates Foundation.

Larry Ellison

Larry Ellison, one of the co-founders of database giant Oracle, is listed by Forbes as the sixth richest person in the world, and America's third richest, with a fortune worth $28bn. According to Forbes his fortunes are continuing to soar thanks to a series of large acquisitions by Oracle, which have seen the firm buying 57 companies in the last five years.

Michael Bloomberg

Michael Bloomberg is the current mayor of New York City, having been elected to a third term last autumn. He is the founder and majority stakeholder of the Bloomberg media company, which provides financial news and information services. Forbes says he is worth $18bn, making him the eighth richest person in the US.

Paul Allen

A university drop out, Paul Allen founded software giant Microsoft with Bill Gates in 1975. Having left the company in 1983 due to illness, he is reported by Forbes to now be worth $13.5bn.

Ronald Perelman

Ronald Perelman attended The Wharton School, part of the University of Pennsylvania and considered to be one of the world's pre-eminent business schools. He has made his $11bn fortune buying beleaguered companies and later re-selling them at an enormous profit. He has invested in a wide portfolio of businesses ranging from Revlon make-up to Humvee maker AM General. Recent forays into the restaurant business include the opening of the Blue Parrot in East Hampton with film star Renee Zellweger.

George Lucas

The Hollywood film director and producer is best known for creating the enormously successful Star Wars and Indiana Jones film series. In 1975 he founded Industrial Light & Magic to create special effects for original Star Wars film - the firm is now Hollywood's most successful special effects company. According to Forbes he is currently worth $3bn.

Ted Turner

Media mogul Robert Edward "Ted" Turner III founded the cable news network CNN, the first dedicated 24-hour cable news channel. Forbes says he is currently worth $1.8bn, though in recent years he has become as well known for his philanthropy as his business success. In 1998 he made a $1bn gift to UN causes, which created the United Nations Foundation, a public charity to broaden support for the UN. He is also the largest landowner in North America, owning two million acres of land.