Oprah, you had us at "family secret."

Oprah Winfrey's announcement that she will stage a surprise reunion on her talk show Monday has prompted speculation about the person's identity. Could it be an unknown sibling? Perhaps even a birth parent who has been in the shadows?

Based on past "The Oprah Winfrey Show" reunions for stars as well as average folk, the episode could provide a tearful catharsis for Winfrey and her guest and — just maybe — boost program ratings as well as attention to her new cable channel.

"I don't have the least idea of what she has in mind," Vernon Winfrey, her father, said Sunday from Tennessee.

He may be alone. Winfrey biographer Kitty Kelley says the on-camera meeting may be with a sister Oprah's never known. Perez Hilton cites the parent theory on his blog.

Harpo, Winfrey's production company, declined to provide details of the show.

Winfrey has not commented publicly about Kelley's 2010 biography, which includes the allegation that Vernon Winfrey isn't her biological father. That claim prompted online speculation by Hilton that she may be reunited with a mystery birth dad.

Winfrey, 57, and various biographies says she was born to unmarried teenagers Vernon Winfrey and Vernita Lee, and raised at different times by a grandmother, her mother, and her father and stepmother in Mississippi, Wisconsin and Tennessee. Two Winfrey siblings, a brother and sister, both have died.

Whatever the answer Monday, Winfrey promised viewers in a promotional spot Thursday that it represents a "miracle."

"I was given some news that literally shook me to my core. This time, I'm the one being reunited," she said. "I was keeping a family secret for months, and on Monday you're going to hear it straight from me."

A clip in the promo, presumably taken from the taped episode, shows the back of a shadowy figure walking onstage.

Winfrey has shared painful and private elements of her life before, willingly or not. Her revelation that she had given birth as a teenager to a son who died shortly afterward came after a relative sold the story to a tabloid in 1990. Winfrey has also discussed her childhood molestation.

The Chicago-based syndicated "Oprah Winfrey Show" is in its 25th and final season. The Oprah Winfrey Network, OWN, launched in January.