Elon Musk and Grimes privately welcomed a third child, the tech mogul confirmed Saturday via X (the social media platform owned by Musk and formerly known as Twitter).

Musk verified the child’s existence after a New York Times review of Walter Isaacson’s forthcoming biography of the SpaceX co-founder mentioned the child by name. It was previously known that the billionaire and the “Miss Anthropocene” artist shared a 3-year-old son and a 1-year-old daughter.

Musk said in an X post that their youngest child’s name is Tau Techno Mechanicus, while Isaacson’s book states that the child’s legal name is Techno Mechanicus Musk and his nickname is Tau.

According to an advance copy of the biography obtained by the Los Angeles Times, the child was born via surrogate in June 2022. The baby boy’s nickname, Tau, was inspired by “the Greek letter representing the irrational number that is equal to two times pi,” the biography explains. The approximate numerical value, 6.28, doubles as a nod to Musk’s birthday, June 28.

More information about Musk’s family is included in Isaacson’s 688-page book, “Elon Musk,” which comes out Tuesday.

The revelation comes less than a week after Grimes responded to an X post from Isaacson promoting the biography and sharing photos of Musk with two of his children. The experimental pop musician later deleted her reply, which read, “tell Elon to let me see my son or plz respond to my lawyer. I have never even been allowed to see a photo of these children until this moment, despite the situation utterly ripping my family apart.”

Representatives for Grimes and X did not respond Friday to The Times’ requests for comment.

Grimes, 35, and Musk, 52, dated on and off from about 2018 to 2022. The pair welcomed a son, X Æ A-Xii, in May 2020, and a daughter, Exa Dark Sideræl, in December 2021.

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