How do you say their names are... in French?

There are a couple of ways to say "their names are..." in French, depending on the context:

Formal:

* Ils s'appellent... (They are called...) - This is the most common and formal way to say it.

Informal:

* C'est... (It is...) - This is a more informal way to say it, often used when introducing people. For example, "C'est Marie et Jean." (This is Marie and Jean.)

Example:

"Their names are John and Mary."

* Formal: Ils s'appellent John et Mary.

* Informal: C'est John et Mary.

You can also use the verb "être" (to be) in a more descriptive way:

* Ils sont John et Mary. (They are John and Mary.)

Remember to choose the appropriate form based on the formality of the situation and your relationship with the people you are talking to.