The lot includes an original photographic proof booklet that contains 27 images.
The unpublished, candid images were taken by Washington Post heiress Katharine Graham at Bunny Mellon's Cape Cod beach shack in August 1961.
The 26-acre Mellon estate, in Osterville, featured 1,000 feet of continuous shoreline. It wasn't far from the Kennedys' Hyannis Port compound.
The Mellon estate was sold in 2013 for nearly $20 million.
RR Auction's executive vice-president says the images are "exceptionally private, unrehearsed moments" that "convey a carefree happiness."
Bidding on the lot begins Feb. 12. A live auction will be held Feb. 19 in Cambridge.
Online: RR Auction, http://www.rrauction.comSuggest a correction