The twins were reportedly born weeks before the Tesla founder and musician Grimes had a baby daughter via surrogate in December 2021.
In April, Musk and Zilis filed a petition to change the twins’ names to “have their father’s last name and contain their mother’s last name as part of their middle name,” Insider reported.
The order was reportedly approved by a judge in Austin, Texas, in May, though the children’s names were not disclosed.
Zilis currently works as director of operations and special projects for Neuralink, which Musk co-founded and where he serves as co-CEO. Neuralink is a Silicon Valley start-up that develops devices to connect humans and computers, known as brain-machine interfaces.
The twins bring Musk’s total number of known children to nine. He is father to Exa Dark Sideræl and X AE A-Xii with Grimes, and had twins and triplets via IVF with his ex-wife, Canadian author Justine Musk, to whom he was married from 2000 to 2008.
One of the twins petitioned in April to have her name changed legally to reflect her gender identity and because she no longer wishes to be “related to my biological father in any way, shape or form.”
The world’s richest man has also been linked to actors Amber Heard and Talulah Riley, whom he married and divorced twice from 2010 to 2016.
This article originally appeared on HuffPost and has been updated.