Grimes and Elon Musk secretly welcomed third child

Elon Musk and Grimes were known to be parents to three-year-old son X Æ A-12, and daughter Exa Dark Sideræl. Photo / WireImage

Elon Musk and Grimes have secretly welcomed their third child into the world.

The on/off couple were known to be parents to three-year-old son X Æ A-12, and daughter Exa Dark Sideræl, 19 months, but a new biography of the Tesla boss - who has fathered eight other children with two women - by journalist Walter Isaacson states they have also had another son, who is officially named Techo Mechanicus but goes by the name of Tau.

The revelation is mentioned in an extensive review of the book, which is simply titled ‘Elon Musk’, by the New York Times newspaper.

The outlet wrote: " Isaacson has ably conveyed that Musk doesn’t truly like pushback.

Some of his lieutenants insist that he will eventually listen to reason, but Isaacson sees firsthand Musk’s habit of deriding as a saboteur or an idiot anyone who resists him.

”The musician Grimes, the mother of three of Musk’s children (the existence of the third, Techno Mechanicus, nicknamed Tau, has been kept private until now), calls his roiling anger “demon mode” — a mind-set that “causes a lot of chaos.”

She also insists that it allows him to get stuff done.

It is unclear if the book contains any more information about the child, including when or where he was born, and representatives for the 52-year-old billionaire and the ‘Genesis’ singer have yet to comment.

Grimes - whose real name is Claire Boucher - recently revealed her older children, who are known as X and Y respectively, share their father’s interests in space and engineering.

Speaking to Wired before the ongoing SAG-AFTRA strike, the 35-year-old star said: “X knows a lot about rockets. It’s crazy. He knows more about rockets than me.

”We had to stop giving him toys, because if they’re not anatomically correct, he gets upset.

”He’s a little engineer, for sure. But his obsession with space is bordering on, ‘Is this healthy?’

”[Y is] a little engineer too. She likes industrial shipping. She’s very strange.”