One would think Paris Hilton's parents have billions at their disposal ... but for some mysterious reason, Rick and Kathy Hilton just took a $10 million equity line out of their Bel Air mansion.


TMZ has obtained financial documents filed recently at the L.A. County Recorder's Office, which show both Rick and Kathy were approved for the credit line ... and whatever they borrow must be repaid in full by 2021.

It's unclear why the Hiltons have any need for such a substantial credit line -- but as one rich person tells us, "It doesn't mean they're broke ... this thing happens all the time."

Attempts to reach the Hilton family for comment were unsuccessful.