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Fortyfour.

She wrote a dissertation "On the Algebra of Logic" with Peirce as the thesis advisor. The dissertation was published in Studies in Logic (Peirce, ed.) in 1883. Due to her studies with Peirce, Ladd became the first American woman to be involved in psychology, mathematics and logic. Since women were not allowed to graduate at Hopkins, Ladd was refused a Ph.D. in Mathematics and Logic, although she was the first woman to complete all the requirements for a PhD at Hopkins. However, Hopkins officially granted her a Ph.D. in February 22, 1926 (44 years after she had earned it) at the age of seventy-eight.

Wikipedia: Christine Ladd-Franklin

(thx @ Simon)