The late Stavros Niarchos amassed one of the "most important collections of Impressionist and modern art in private hands.
He was suspected as being the anonymous buyer of Vincent van Gogh's "Self-Portrait", at a November 1998 Christie's auction; it sold for $71.5 million.
Warhol completed these works in 1985, using silkscreens made from CAT-scan films of the skull of Philip Niarchos, who commissioned the artist to paint his portrait.
Warhol shares details of the dysfunctional relationship Niarchos had with the divorced and widowed socialite Barbara (née Tanner) de Kwiatkowski.
Her married name was Barbara Allen at the time of her relationship with Niarchos; now she is the widow of Henryk de Kwiatkowski.