Katie Holmes and Tom Cruise keep their marriage ticking over by writing down their marrital slip-ups, according to a U.K report.

The showbiz couple - who are currently in Tokyo for the premiere of Valkyrie - have reportedly vowed to put pen to paper when the going gets tough in their marriage.

A source tells the Daily Mail, "Katie has to confess to something as minor as forgetting to tell him she has met with a friend.

"If she commits a transgression against the moral code of their marriage, she has to tell Tom in writing, giving full details of the time and place and what happened."

Meanwhile Katie - who is looking increasingly pail and frail - says that she doesn't let the rumours about her family stand in the way of her being a good wife, mother or actress.

See Katie filming The Extra Man below