Elon Musk and Grimes have secretly welcomed a third baby, according to a new biography based on the Tesla CEO. 

In the the book, which is set to be released on Sept. 12 and is written by journalist Walter Isaacson, the former couple became parents again to a third child, a son named Techno Mechanicus. His nickname is reportedly Tau. Other details including when or where the baby was born have not yet been disclosed. 

Musk and Grimes (born Claire Boucher) welcomed their first child, son X Æ A-12, whose name was later changed to X Æ A-Xii and is nicknamed “X” in May 2020. Less than a year later, the pair welcomed a daughter via surrogate named Exa Dark Sideræl, who is nicknamed “Y," after they had already broken up. 

Musk is also a father to the six children he shares with his ex-wife, author Justine Wilson, as well as twins, son Strider and daughter Azure, with venture capitalist Shivon Zilis in November 2021 — weeks before he and Grimes welcomed their daughter.  

After the news broke about the pair's third baby, Elon took to X (Formerly known as Twitter) to offer some clarification behind the newborn's unique name. "Tau Techno Mechanicus. Circumference/Diameter," he wrote. As for their eldest son, X Æ A-Xii, Grimes explained the meaning, revealing that X stands for "the unknown variable," while Æ is "my elven spelling of Ai (love &/or Artificial intelligence)," and A-12 refers to an aircraft which is the "precursor to SR-17 (our favorite aircraft)."

Exa, meanwhile, refers to the supercomputing term exaFLOPS, while Dark symbolizes “the unknown."

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