Billionaire tech entrepreneur Elon Musk had twins last year with an executive at one of his companies, online outlet Insider reported, citing Texas court documents.
The babies' mother, Canadian Shivon Zilis, is an executive at Neuralink, Mr Musk's brain implant maker, and has worked at several of his other companies, including OpenAI and electric car manufacturer Tesla, Insider said.
She and Mr Musk, who became the richest person in the world last year, in April filed a petition with a Texas court for the children to "have their father's last name and contain their mother's last name as part of their middle name", Insider reported, referring to court documents obtained by the publication.
The petition was granted in May, the site said.
The babies, whom Insider report were born in November, arrived just weeks before Mr Musk and music artist Grimes had their second child via surrogate.
They welcomed a baby girl named Exa Dark Sideræl Musk ― although her parents will mostly call her Y.
Mr Musk is known to have fathered 10 children, one of whom died shortly after birth. He has long advocated for large families and warned that the US birth rate is below "sustainable" levels.
On Thursday, he tweeted: "I hope you have big families and congrats to those who already do!" and suggested Tesla could make a "highly configurable Robovan", presumably to move large numbers of children around.
Last month, one of his children, who recently turned 18, filed a petition in a California court to change her name and gender identity to female.
She cited "the fact that I no longer live with or wish to be related to my biological father in any way, shape or form" as one of the reasons for the name change, according to the court document.
The chief of Tesla and SpaceX, Mr Musk early this year launched a now-stalled $44 billion bid to buy Twitter, with Insider reporting that Ms Zilis's name has come up as an option for who might eventually run the social media company.
Tesla, which has relocated to Texas from California, has faced a series of lawsuits alleging discrimination and harassment against black workers, as well as sexual harassment.
And in May, the South African-born Mr Musk denied allegations that he groped and exposed himself to a flight attendant six years ago, with news outlet Insider reporting that SpaceX paid a woman $250,000 in 2018 to settle a misconduct claim against him.
AFP contributed to this report