(Reuters) - Never believe Federal Reserve Chairman Ben Bernanke doesn't have a sense of humor.

"I wrote recently to inquire about the status of my leave from the university, and the letter I got back began, 'Regrettably, Princeton receives many more qualified applicants for faculty positions than we can accommodate,'" Bernanke said in remarks prepared for delivery on Sunday to new Princeton University graduates in his commencement speech.

"Note to journalists: This is a joke," said a footnote attached to the text provided to reporters.

Behind the joke, and Bernanke's care in making sure journalists didn't miss it, is the mounting obsession over who may take charge of U.S. monetary policy for the world's largest economy after January 31, 2014, when Bernanke's second term expires.

Bernanke has said little about his future at the Fed, where he took over as chairman in 2006.

Talk that he is reluctant to accept a third term has been swirling for months, and speculation that he plans to leave the post got a boost in late April when the Fed confirmed he will skip the annual Jackson Hole monetary policy symposium in August due to a scheduling conflict.

Bernanke's absence would mark the first time in 25 years that a Fed chairman has not attended the conference, which draws top central bankers from around the world and is periodically used as a setting to preview important policy shifts.

Fed Vice Chair Janet Yellen is most often mentioned as his likely successor.

Bernanke, who led the Fed's response to the financial crisis and the Great Recession that followed, taught economics at Princeton before joining the Fed Board of Governors in 2002. His public service leave from Princeton expired in 2005. He became Fed chairman the following year.

Bernanke, who did not address monetary policy or the outlook for the economy in his speech, advised students to be on the lookout for the unexpected twists and turns in life.

"Life is amazingly unpredictable; any 22-year-old who thinks he or she knows where they will be in 10 years, much less in 30, is simply lacking imagination," he said. "Don't be afraid to let the drama play out."

(Reporting by Ann Saphir; Editing by Diane Craft)

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