John Travolta and his late wife, Kelly Preston, had three children together … but their youngest kid is biologically a Presley … this according to a bombshell legal document.

Priscilla Presley‘s former business partner, Brigitte Kruse, is currently suing Priscilla’s son, Navarone Garcia, for breach of contract … but what’s way more interesting in new legal docs filed in the case is that Kruse alleges John and Kelly used Riley Keough‘s eggs to give birth to their son, Ben.

In the papers, Kruse alleges Michael Lockwood — Lisa Marie Presley‘s ex-husband — told her Kelly had been unable to bear her own children. Kruse claims John and Kelly previously used Lisa Marie’s eggs to get pregnant … but they no longer wanted Lisa’s eggs because they did not want “eggs with heroin” on them.

It’s unclear whether Lisa Marie’s eggs ever produced a child.

However, Kruse claims a deal was orchestrated where Riley — one of Lisa Marie’s daughters — gave her eggs to Travolta, so that Kelly could give birth to Ben, who is 15 now. Kruse claims Riley was given an old Jaguar and paid between $10,000 – $20,000 for the deal.

Kruse claims Lockwood was incessant, telling her John “needed to help salvage his career amid claims of sexual assault against other men.”

The docs include a handwritten note with the words “CA fertility partners,” “Ben Travolta,” and “Kelly Preston carried baby” scribbled on it.

They also include a text exchange where it looks like Ben Travolta is referred to as Priscilla’s “beautiful great-grandson.”

Priscilla’s attorneys Marty Singer and Wayne Harman tell TMZ … “After losing motion after motion in this case, and unsuccessfully seeking to have Presley’s counsel of record, Marty Singer, disqualified from representing her in this matter, Brigitte Kruse, Kevin Fialko, and their co-conspirators have demonstrated that there is no bar too low, no ethical line that they are unwilling to cross in an effort to cause further pain to Priscilla Presley and her family.”

They added … “In a completely improper effort to exert undue pressure on Presley to retract her legitimate, truthful claims, Kruse and her co-conspirators have also sued Presley’s son, cousin, and assistant. These recent outrageous allegations have absolutely nothing to do with the claims in this case. The conduct of Kruse, Fialko, and their new lawyers (they are on their fourth set of attorneys) is shameful, and it absolutely will be addressed in court.”

Jordan Matthews, of Holtz Matthews LLP, tells TMZ … “Brigitte Kruse and Kevin Fialko are heartbroken that they have been forced to file their most recent amended complaint. They have been accused of horrific acts that are not supported by any evidence whatsoever.”

Matthews says … “This amended complaint is about revealing the truth, correcting the record, and providing evidence of the work that was done in good faith to bring peace, resolution, and stability to the Presley family, despite being exposed to constant volatility.”

We reached out to a rep for Travolta … so far, no word back.

A viral video that appears to highlight radio DJs in Arkansas having sex in the control room — unbeknownst to them, live on air — is a total fake, TMZ has learned.

Here’s the deal … a video clip circulating on social media shows a car radio tuned to Power 92.3 Jams in Little Rock … and it sure sounds like the DJs are going at it on a hot mic. Naturally, the wild and hilarious situation — which people assumed was legit — spread like wildfire.

Turns out, those sharing it are the ones who got played … ’cause Keith Liesmann, regional VP/market manager for the station’s parent company Cumulus, tells TMZ … “We are aware of a video circulating online and want to be absolutely clear: it is fabricated and untrue. Nothing depicted in the video occurred, and it has no connection to Power 92.”

Nick Reiner appeared noticeably different in the days leading up to the deaths of his parents, Rob and Michele Reiner — coming off as “drained” and “abnormal” … according to gas station employees who say they’ve known him for years.

TMZ spoke with multiple employees at a Sinclair gas station in Brentwood, a spot they say the Reiner family frequented regularly.

One employee tells us they last saw Nick about two weeks ago, recalling he looked tired and was unusually standoffish.

At the time, Nick purchased a pack of Marlboro Lights, which employees say was routine for him. They add that he did not appear to be under the influence of drugs or alcohol.

The New York Post obtained surveillance video showing the 32-year-old walking through the gas station in the early hours of Sunday morning … just hours before Rob and Michele were killed.

We’re told the Reiners were loyal customers, with one employee saying they’d known Nick for  15 years … and the news has left staff stunned.

Nick Reiner was arrested hours after his parents were found stabbed to death Sunday … he’s been charged with two counts of first-degree murder.