I think they’re just called subtitles, but I’d be happy for someone to prove me wrong because they deserve a more interesting term.
And just because no one else has said it I’ll ad one of my favorite book titles: “Frankenstein; or, The Modern Prometheus.” Shelley showed off some of her writing chops right there on the cover. And its one of the only times I can use a semicolon and know that I’m doing so correctly.