Duke Leto and Lady Jessica, mother and father of Paul Atreides, were never married.
Instead, She was his concubine.
My limited understanding of the practice is that a concubine was a legally accepted bonded female taken in to provide birth to a child in lieu of a barren wife…not just some piece of ass a guy keeps on the side.
If this is true, Who IS Duke Leto married to?
He never married. Jessica was his one twue luve, but the politics of the Dune Universe meant that Dukes would generally marry Dukettes from rival Great Houses for purely political purposes.
Was she called a concubine?
I’d call her a mistress myself, though even that usually implies the guy is married.
How about “live in girlfriend?” “Partner?” “Life partner?”
-FrL-
CMC fnord!
Oddly enough, I just started reading a copy of Dune I found in an airport bookstore. It was legal for the duke to marry his concubine (and they mention another duke who does just that), but the duke did not marry his concubine because he wanted to hold onto the promise that other noble houses could ally themselves with him through marriage.
Frylock: She is officially known in the books as his concubine. She’s also Bene Gesserit, told only give birth to females of the Duke’s line, under strict orders. (They have this ability, to affect the gender of the child they concieve.) They planned to marry the female of Leto’s line to a male of House Harkonnen in their quest to make the Kwisatz Haderach.
For what it’s worth, Dictionary.com pretty much defines a concubine as some piece of ass kept on the side. It could mean a secondary wife, but its primary meaning seems to be “mistress.”
crowmanycloudsIf you have read much of the series see the spoiler, if not, don’t.
It is said that Jessica’s father is Baron Vladimir Harkonnen, and that it was due to a “clever seduction” that she was concieved.
Not just said. It was confirmed in the books, else Alia wouldn’t be able to have Harkonnen among her ancestral voices.
Another angle to the story, I think, is that if Leto had a son by a legal wife, Paul would be disinherited. That’s the other reason he never married.
Paul did the same thing with Chani, kept her as a concubine and mother of his children, though he had a political (and deliberately childless) marriage to Irulan to cement his claim as Emperor. The last line of the book Dune has Jessica saying to Chani, “we who carry the name of concubine–history will call us wives.”
Wasn’t Irulan technically the Empress-regnant and Paul the Emperor-consort ruling as regent in her name? 
Of course then why would Leto II ascend after Paul went into the desert if Irulan was still alive?
Are you sure? Paul was the the dukes designated heir. AFAIK, the duke had the power to name anyone as the official successor to the duchy. I was always under the impression that even if teh duke had married for political reasons, Paul was still going to be the next head of the Atreides.