A drunken Spencer Tracy (search) once smacked screen legend Katharine Hepburn (search) across the face, but she refused to end her celebrated affair with her married co-star, a new biography reveals.

"If I had left, we both would have been miserable," Hepburn told biographer A. Scott Berg. The book, Katharine Remembered, goes on sale this morning.

According to the biographer, Tracy whacked Hepburn in the face one "fiendish night" with the back of his hand when she tried to put him to bed at the Beverly Hills Hotel — but he was too drunk to remember the incident, and she never reminded him of it.

In an effort to get Tracy help, Hepburn thought of getting him into Alcoholics Anonymous (search), but decided against it because she didn't believe it would stay a secret.

The biography is based on conversations between Hepburn and Berg over more than 20 years. Berg respected Hepburn's wishes and withheld publication of Kate Remembered until her death last week at age 95.

Among other juicy revelations:

* The star, noted for her dignity and class, once posed for nude photos in college and she always feared they would surface.

* Hepburn's longtime friend, Irene Selznick, drifted apart from her after Selznick suspected Hepburn of a lesbian relationship with a young woman who had been staying at the star's East 49th Street apartment.