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
A look at SpaceX in Texas - in pictures
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A sign for Starbase, Elon Musk's private spaceport in Boca Chica, Texas. All photos: Willy Lowry / The National -
Three SpaceX rockets stand ready at Starbase in Boca Chica. -
A SpaceX employee works high above the ground on a rocket at the SpaceX launchpad. -
A SpaceX rocket rises in the distance. -
A sign at Boca Chica Beach warns of sea turtle nesting. -
Three Starship rockets stand next to a booster. The reusable rockets are at the centre of SpaceX's plans for the future. -
A sign warns passers-by against entering the area around SpaceX's launchpad. -
A sign along Texas State Highway 4 warns drivers to watch out for local wildlife. -
A SpaceX security guard questions a driver at the entrance to the launch zone. -
Amanda Maul and her son, Connor, drove all the way from Dallas to see SpaceX. -
SpaceX's Starbase facility sits near the Gulf of Mexico in a remote patch of southern Texas. -
Boca Chica Beach, looking south towards Mexico. It is one of the last free public beaches in southern Texas. -
Workers prepare to move a Starship rocket. -
SpaceX's Starbase rises in the distance. -
A sign marks the lines where the Boca Chica National Wildlife Refuge begins along the Texas State Highway 4. -
Flowers grow on the dunes of Boca Chica Beach near a SpaceX tower. -
A person walks in the Boca Chica Wildlife Refuge next to Starbase. -
A view of Starbase from Boca Chica. -
Flowers grow on the dunes of Boca Chica Beach near a SpaceX tower. -
A sheriff's car parks near SpaceX in Boca Chica. -
A sign along a motorway reads: 'We love Elon'. -
An exterior sign for Starbase. -
Cars parked outside of Starbase in Boca Chica. -
Some SpaceX employees stay in Airstream trailers parked near the complex in Boca Chica. -
A sign welcomes visitors to Boca Chica Village. -
A renovated home in Boca Chica. -
A Tesla charges in the driveway of a home in Boca Chica. -
A shuttered home in Boca Chica. Elon Musk, the world's richest person, lives in the tiny unincorporated village. -
One of two streets in Boca Chica Village, which leads straight to Starbase. -
A renovated home in Boca Chica Village. SpaceX has been slowly buying up property in the area for years. -
A home in Boca Chica Village. The community used to be mainly made up of retirees, but it is now comprised almost entirely of SpaceX employees. -
A Tesla charges in the driveway of a house in Boca Chica. -
A sign on a house in Boca Chica Village warns passers-by they are on private property. -
A white Tesla sits in the driveway of a recently renovated home in Boca Chica Village. -
A sign on a house in Boca Chica Village warns passers-by they are on private property.
