PORTLAND – A legal case in which the daughter of an IBM executive who built the company into a computer giant adopted her same-sex partner to ensure her financial security is playing out in a complex legal dispute that is taking place in Maine courtrooms.

Olive F. Watson legally adopted her longtime partner, Patricia A. Spado, in 1991 in probate court in Rockland. Watson is the daughter of the late Thomas J. Watson Jr., who was the CEO of IBM from 1956 to 1971.

At stake is a multimillion-dollar trust fund. At the same time, the case raises questions about whether same-sex couples can use Maine’s adoption laws to establish legal rights, including inheritance, denied them because they can’t marry.

The relationship between Spado and Watson ended in 1992, and their adoption resurfaced in Maine in 2005 when an attorney for the Watson family trust filed a motion to annul the adoption. A judge granted the motion on a technicality.

The Maine Supreme Judicial Court last week overturned the ruling and sent the case back to Probate Judge Carol Emery with instructions to judge the case on its merits.

The dispute is scheduled for a pretrial conference in Knox County Probate Court on Feb. 7. Another dispute is pending in a court in Connecticut, where Thomas Watson died in 1993.

Adult adoption is legal but rare in Maine. It sometimes occurs when an adult child wants to take the name and formalize a bond with a stepparent who raised him or her or to create a path for inheritance between unrelated people.

That was apparently the case when Olive Watson legally adopted Spado 16 years ago. At the time, they were in their 40s and had been together for 14 years.

After they separated, Olive Watson eventually paid Spado about $500,000. Watson’s attorneys say the money was intended to release the family from any rights Spado would have gained through the adoption. But Spado in court filings produced a letter signed by Watson shortly after their breakup that said she would not initiate any action to annul the adoption.

The case took a turn when Olive Watson’s mother died in 2004. Upon the elder Watson’s death, her 18 grandchildren became eligible to receive income from two trusts until they turned 35, at which time they would receive the principal outright.

But a few months later, an attorney representing Spado notified the trust that there was a 19th grandchild – Patricia Spado – who also was entitled to a share of the trust.

The claim was challenged successfully by trustees in a Connecticut probate court. They argued that Thomas Watson knew Spado as his daughter’s lover, not as his grandchild, and that he did not intend her to benefit from the trust. The decision is now on appeal in the state’s Superior Court.

In Maine, trustees filed a petition in 2005 seeking to annul the adoption on the ground that the probate court lacked jurisdiction to grant the adoption because Spado didn’t live in Maine. At the time of the adoption, Spado and Watson were vacationing on North Haven, where Watson owned property.

The trustees further argued that Watson and Spado never intended to establish a normal parent-child relationship because the two were engaged in a same-sex relationship and had obtained the adoption to benefit Spado financially.

Through their attorneys, Spado and Watson declined comment. The attorneys also declined comment.

But in court documents, Stephen Hanscom, a Rockland attorney who represents the Watson family, argued that it would be a public-policy disaster to let the adoption stand. He said it would have been illegal in New York and Connecticut.

“A decision that Olive Watson’s adoption of Patricia A. Spado was valid, despite both parties’ lack of residency in Maine and their sexual relationship, would have far-reaching and national implications,” Hanscom wrote.

“It would mean that parties from anywhere in the country could come to Maine and adopt their sexual partner to manufacture inheritance rights for that person, even if neither party was a Maine resident and their state of residency would not have permitted the adoption,” he wrote.

Advocates for same-sex marriage say that gay and lesbian couples aren’t likely to pursue adoption in any state because the arrangements do not always work as intended.