Letter from Kennedy sister up for auction says Robert Kennedy and Marilyn Monroe were an 'item'

Is there finally written proof that Robert Kennedy and Marilyn Monroe were paramours?

A letter up for auction from Kennedy sister Jean Smith to the sex siren contains the titillating line: "Understand that you and Bobby are the new item! We all think you should come with him when he comes back East!"

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The two-page missive was written in the 1960s, when Marilyn was often seen in the company of Peter Lawford, a Kennedy brother-in-law and famously sang "Happy Birthday" to John F. Kennedy in a breathy voice and a barely there dress.

See images of the infamous actress:

That beaded gown also is part of a trove of Monroe personal items up for sale and it's expected to fetch as much as $3 million at a Beverly Hills auction on Nov. 17.

Rumors of alleged affairs between Monroe and both brothers have swirled for decades.

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The troubled actress died in 1962 from a drug overdose. JFK was assassinated a year later and RFK met a similar fate six years later.

Also up for bid will be her 1947 contract with Twentieth Century Fox, a recipe for stuffing, her final checkbook, a pair of strap sandals she wore in 1943 while still named Norma Jean and a negligee from the movie "Niagara."

The letter was discovered among papers Monroe left to her famous acting coach, Lee Strasberg. His son, David, found them packed into suitcases and cupboards.

Watch: Marilyn Monroe's Iconic Dress Worn For JFK's Birthday Could Fetch $3 Million at Auction