A little more than a decade had passed since Einstein and Elsa met when Albert fell under the charm of Betty Neumann, the niece of a friend. By one account, Elsa allowed the two to meet twice a week so Albert wouldn't "sneak around." The affair ended in 1924, but Einstein continued to have romantic liaisons with other women throughout his marriage and after Elsa's death in 1936.
Albert Einstein Archives, The Hebrew University of Jerusalem, Israel 8

Dearest Betty!

Uncle and Aunt Hans told me that you are now divorced again. You will be breathing a sigh of relief, and I suspect it from you. You jumped into that relationship like a frog into water. Dear Betty, have faith in your Uncle Hans and Aunt Minna, who love you. They will help you to find the right path. Oh, Betty, if I wasn't wedged, as I really am, then you wouldn't have to spend so much time searching! I was already happy when I knew you were somewhere in the neighborhood or if I could sometimes see your lovely smile. But Fate is cruel, even to someone like me who is envied by many.

As I musn't run after you, it is my everlasting hope to meet you accidentally, but this happens so rarely, and not even by accident. Dear Betty, laugh at me, the old donkey, and find somebody who is 10 years younger than me, and who loves you just as much as I do.

I embrace and kiss you
A. Einstein