When Olive Watson and Patricia Spado discovered they could not legally marry in Maine in 1991, they decided to take a less conventional route to cement their rights.

Ms Watson adopted Ms Spado, naming her as the sole beneficiary of her will and cementing her rights of inheritance.

The couple split in 1992, but Ms Spado is claiming her share of inheritance as an official “grandchild” of Ms Watson’s family.

This could be just another story about middle America, where gay couples denied civil union or marriage rights jump through loopholes in the law, which can present problems when the relationship turns sour.

But Olive Watson is the granddaughter of famous entrepreneur Thomas Watson Jr, and an heiress to the IBM fortune.

According to court documents, at the time the adoption took place Ms Watson was worth $10m.

“It certainly illustrates the lengths that many gay and lesbian couples have to go to protect their relationships,” Carisa Cunningham of Gay and Lesbian Advocates and Defenders told AP.

For 14 years Ms Watson and Ms Spado shared homes and bank accounts, and even bought a dog together. They are reported to have only spent five nights apart in all that time.

When the couple separated, Ms Spado was paid a settlement of half a million dollars.

Thomas Watson Jr, who built IBM into the giant corporation it is today, died in 1993, apparently unaware of the adoption. His wife died in 2004.

With the deaths of both parents, their grandchildren became eligible for trust-fund payouts once they reach the age of 35.

But when Ms Spado’s lawyers notified the trusts that she was a potential beneficiary as a grandchild, the family challenged the claim. The size of the estate at stake has not been estimated in court documents.

Ms Watsons lawyers are currently before the Maine courts trying to negate the original adoption on the grounds that it was fraudulent. However, Ms Spado’s lawyers have produced a note written by Ms Watson after the initial settlement saying “I have not and that I shall at no time initiate any action to revoke or annul my adoption of you.”

To this date, gay marriage is legal only in Massachusetts. Six other states and Washington DC recognise same-sex unions and offer civil unions or partnerships.