YAHOO’S billionaire founders found another fortune on Friday – not that they were looking.

Jerry Yang and David Filo’s wealth increased by 50% as Microsoft began a $44.6 billion (£22.6 billion) hostile bid for their search engine. Filo, 41, and Yang, 39, are Yahoo’s biggest insider shareholders.

Filo has 78.2m Yahoo shares, while Yang has 52.8m, the Securities Exchange Commission says, and both hold millions of options and indirect shares.

Microsoft’s cash and shares offer values Yahoo at $31 a share so Filo’s stake would be worth more than $2.4 billion, without his options and other shares. Yang’s stake, without options, would be over $1.64 billion. The day before the bid their stakes were worth about $1.5 billion and $1 billion respectively.

The pair, who founded Yahoo while at Stanford University in California, used to be richer. In 2000, before the dotcom bust, Forbes magazine estimated Filo had a net worth of $6.5 billion, and Yang was worth $6.4 billion.

Dotcom companies have bounced back since the crash, but Yahoo has been plagued by accusations of mismanagement and fierce competition from Google and newcomers such as Facebook and MySpace.

Yahoo shares closed on Friday at $28.35.

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