Tom and Julia are "husband and wife" or "_a_ husband and wife"

I saw this sentence in a book:

Tom, 45, and Julia, 40, are husband and wife.

It sounded natural to me not to use the article "a" before "husband and wife". But I'd like to know the grammatical explanation.

Solution 1:

It's an idiom.

In idioms articles are often omitted.

According to Merriam-Webster Dictionary:

 idiom

 husband and wife

: a married couple

// They were husband and wife for almost 60 years.

And have a look at this.

It adds some statistics to the previous information.

By the way, according to Oxford English Dictionary it is a hyphenated adjective:

"husband-and-wife

ADJECTIVE

Relating to or involving a husband and wife; consisting of a husband and wife."

In this case there's no problem of using or not using the indefinite article here because adjectives as predicatives are used without articles.