(The Hill) – Tesla CEO Elon Musk reportedly had twins, his eighth and ninth known children, with a top executive at one of his companies last year.

Court documents obtained by Business Insider on Wednesday showed that Musk and the children’s mother, Shivon Zilis, filed a petition to change the twins’ names in order to “have their father’s last name and contain their mother’s last name as part of their middle name.” 

The order was then approved by a federal judge in May. Zilis purchased a home in the Austin, Texas area last August, nearly three months before the twins were born, according to Insider. Musk also has a waterfront estate in the city.

The twins were born weeks before Musk and his then-girlfriend, the musician known as Grimes, gave birth to their second child via surrogate last December. 

Zilis, a 36-year-old Yale University graduate, began her career at companies including as IBM and Bloomberg Beta before joining Tesla in 2017 as a project director. 

The Ontario, Canada native, who was named onto the Forbes’ 30 Under 30 list in 2015, reportedly first met Musk that year through her work for his nonprofit OpenAI.

The 51-year-old CEO has been an outspoken advocate for more childbirth, expressing fear of a possible fertility crisis in the U.S. and touting his own success with childbirth.

“Contrary to what many think, the richer someone is, the fewer kids they have,” Musk said in a Twitter thread. “I am a rare exception. Most people I know have zero or one kid.” 

“I mean, I’m doing my part haha,” Musk said in a follow-up tweet in the thread last month. 

Musk has yet to close on his big to acquire social media platform Twitter, however Insider noted that Musk had Zilis on the short-list of individuals to run the company should the deal go through.