Sandy Weill, the man who created Citigroup, Larry Ellison, founder of the software giant Oracle, and Pierre Omidyar, founder of eBay, are among the billionaires making a public pledge to give away more than half their fortunes.

Forty of the world's richest families have signed up to an initiative started in June by Microsoft's founder Bill Gates and Warren Buffett, the investment guru, aimed at promoting philanthropy among the super-rich.

Gates and Buffett are the two richest men in the United States, with a combined wealth of US$100 billion ($137 billion), and are firm friends.

The pair - who are both putting the bulk of their fortunes into the Bill and Melinda Gates Foundation, which fights disease in the developing world and promotes education in the US - have been searching for supporters for what they call "The Giving Pledge" after becoming concerned that the recession cut into charitable giving.

Billionaire signatories have to pledge that they will give away a majority of their wealth either in their lifetime or on their death, and write a public letter explaining why. They also agree to meet annually to discuss the most effective way of making a difference.

In a letter explaining his pledge, Weill and his wife Joan wrote that "shrouds don't have pockets". George Lucas, the creator of Star Wars, said he was dedicating his fortune to improving education.

"When I was in high school, I felt like I was in a vacuum, biding time," he said.

"It's scary to think of our education system as little better than an assembly line with producing diplomas as its only goal. Once I had the means to effect change in this arena, it became my passion to do so."

Omidyar said he has been increasingly interested in philanthropic causes since eBay went public in 1998.

"In a matter of days we went from making a modest living to landing a spot on Forbes' list of richest Americans. It was a surreal experience. But the responsibility we felt for being good stewards of that wealth was genuine - and has grown ever since," he said.

Other signatories include media mogul Barry Diller and his wife, the designer Diane von Furstenberg.

- Independent