UPDATE 1-Comedian Robin Williams' wife files for divorce

(Recasts 1st paragraph, adds details from court papers)

SAN FRANCISCO, March 26 (Reuters) - Comedian Robin Williams' second wife, whom he met when she was his nanny, has filed for divorce citing irreconcilable differences, court documents showed on Wednesday.

Marsha Garces Williams filed her petition in a San Francisco court late last week. The couple has been separated since Dec. 31, and she is requesting joint custody of their 16-year-old son, Cody, according to the filing.

The couple have been married since 1989 and have two children. Their daughter, Zelda, is 18 years old. They first met when she took care of Williams' son with his previous wife, Valerie Valardi.

Williams, 56, rose to fame in the 1970s playing the wildly comic space alien Mork in the hit television show "Mork and Mindy," and is known for his frantic, zany standup comedy.

Over the years, Williams has built a successful career in movies performing in comedies, dramas, big Hollywood flicks and low-budget art house films.

Williams won an Oscar for best supporting actor playing a psychologist in coming-of-age drama "Good Will Hunting." He has been nominated three times for the best actor Academy Award for his roles in "Good Morning, Vietnam," "Dead Poets Society" and "The Fisher King."

Garces Williams has worked as a producer on her husband's films and co-owns their production company, Blue Wolf Productions. Her attorney, Robert Kaufman, was not immediately available to comment and Williams' spokeswoman declined further comment. (Reporting by Amanda Beck, Editing by Bob Tourtellotte and Todd Eastham)