It’s no secret that the future Supreme Court justices Sandra Day O’Connor and William Rehnquist dated when they were both studying law at Stanford University in the early 1950s.

But a forthcoming biography of O’Connor reveals that Rehnquist tried to take their relationship to the next level ― unbeknownst to their families and friends until now.

While researching the original female Supreme Court justice for his book First, slated for release in March 2019, author Evan Thomas discovered letters between the two that indicated Rehnquist continued to romantically pursue O’Connor months after they had broken up, NPR reported.

ASSOCIATED PRESS Sandra Day O'Connor in a 1950 Stanford University yearbook photo and William Rehnquist in a 1948 Stanford yearbook photo.

Sandra Day, as she was then known, started classes at Stanford Law in 1949 when she was 19 years old. Rehnquist, who had served in the military during World War II, was her 26-year-old classmate when they met that year and began dating.

Several months later, Day decided to break it off with the future chief justice of the highest court in the land. She began dating John O’Connor, whom she would marry in 1952 ― but not before Rehnquist sent her a letter asking for her hand in marriage.

“To be specific, Sandy, will you marry me this summer?” Rehnquist wrote. She replied no, though the two remained close friends throughout their lives. He went on to marry Nan Cornell in 1953.

Sandra Day O’Connor spoke at Rehnquist’s funeral in September 2005. She had announced her retirement from the Supreme Court that summer while the chief justice had decided to stay on the bench even though he had thyroid cancer.

“If you valued your money, you would be careful about betting with the chief ― he usually won,” O’Connor told mourners at St. Matthew’s Cathedral in Washington. “I think the chief bet he could live out another term despite his illness. He lost that bet, as did all of us, but he won all the prizes for a life well-lived. We love you, William Hubbs Rehnquist.”

Read more about the secret proposal at NPR.