Image copyright Stephens County Jail Image caption Each woman could face up to 10 years in prison

A mother and daughter are facing incest charges in the state of Oklahoma after authorities learned they were legally married earlier this year.

It is unclear what motivated Patricia Ann Spann, 43, and her daughter, Misty Velvet Dawn Spann, 25, to wed.

Under state law, marriage to a close relative is considered incest, regardless of whether a sexual relationship exists.

Investigators also found Patricia Spann married her son in 2008.

He filed for an annulment 15 months later, citing "incest", say authorities.

Police discovered the marriages late last month during a child welfare check-up.

Patricia Spann told investigators she had lost custody of her biological children as a young mother and only came into contact with her daughter two years ago.

She said they immediately "hit it off", according to a court affidavit cited by the Lawton Constitution newspaper.

Patricia Spann said the couple believed the union was legal, since she was not listed as the biological mother on her daughter's birth certificate.

Each woman faces up to 10 years in prison if convicted.