Just when stepparents overcome the emotional turmoil of remarriage and begin to form a stable relationship with their stepchildren, they often face an unexpected problem: They have few, if any, legal rights as parents.

Stepparents cannot legally sign a field trip permission slip. Nor can they authorize medical treatment for a child. "A stepparent does not have any legal rights that are greater than any other stranger," says Sanford Ain, an attorney in the District who specializes in family law.

In some instances, the biological parent who no longer lives with the child may grant the stepparent authority to act in his or her place. But such arrangements are by no means the norm.

Stepparents also have no right of visitation or custody in cases of separation, divorce or death. Take, for example, a woman who marries a man who has custody of his two children. The couple stays together for seven years, the woman forms a close relationship with the children, but then the couple divorces.

"Theoretically, the wife is entitled to one-half interest in the marital home but no right {in the majority of states, including Maryland, Virginia and the District} to see her stepchildren," says Richard Victor, a lawyer in Birmingham, Mich., who serves as legal adviser to the Stepfamily Association of America Inc.

Some marriage and family therapists, like Thomas Seibt, who practices in Burbank, Calif., Calif., encourage adults to write a living trust or will about responsibility for their children.

"Without these instructions, a biological parent who has been completely out of the picture can show up on the scene {after a death} and say, 'These are my kids. Adios,' " Seibt says. "The children can be taken out of a very loving relationship and be placed with a biological parent they don't know."

Living wills are a good idea, says Ain, but "are not legally binding."

"Courts are certainly interested in what a deceased parent's wishes were for his or her children," says Ain, but are not compelled to follow those wishes.

In order for a stepparent to win custody of stepchildren after a death -- or, in some cases, after a divorce -- there would have to be very strong evidence that the surviving biological parent is unfit and that the children would be traumatized by switching households. There is a chance, says Ain, that a court would award custody to a stepparent in an exceptional case. But the odds, he says, "are very, very unlikely."Granting Permission

A medical authorization form permits a stepparent to act as a parent in approving a child's medical care. For a free copy, send a stamped, self-addressed envelope to Richard S. Victor, 555 S. Woodward Ave., Birmingham, Mich. 48011.