One of the 20th century’s most illustrious couples, Frida Kahlo and Diego Rivera were known to have an open relationship, preferring not to hide their extramarital affairs.

But there was one tryst Kahlo wished to keep a secret, telling her Spanish paramour José Bartoli, whom she met in New York while recovering from one of her many corrective surgeries, to address her as Sonja in their love letters. Those letters remained private long after Kahlo’s death in 1954, only discovered by Bartoli’s family upon his passing in the 1990s and not seen by the public until now.

The correspondence from Kahlo to Bartoli — roughly 25 letters, written in Spanish — include enclosures ranging from pressed flowers and beads to photographs and an original drawing of a sleeping cat. Poetically and passionately composed, they not only illustrate Kahlo’s unwavering love for Bartoli, but also shed light on several important works, such as Tree of Hope (1946), as well as her post-surgery relationship with her husband, and an unknown pregnancy. 

The letters and their enclosures were auctioned off today at Doyle New York, where they remain on view.