During the 1980s, Einsteins private letters revealed that he had an illegitimate daughter with a fellow former student Mileva Marić.
Mileva gave birth to a daughter at her parents’ home in Novi Sad. This was at the end of January, 1902 when Einstein was in Berne. It can be assumed from the content of the letters that birth was difficult. The girl was probably christianised. Her official first name is unknown. In the letters received only the name “Lieserl” can be found.
The further life of Lieserl is even today not totally clear. Michele Zackheim concludes in her book “Einstein’s daughter” that Lieserl was mentally challenged when she was born and lived with Mileva’s family. Furthermore she is convinced that Lieserl died as a result of an infection with scarlet fever in September 1903. From the letters mentioned above it can also be assumed that Lieserl was put up for adoption after her birth.
In a letter from Einstein to Mileva from September 19, 1903, Lieserl was mentioned for the last time. After that nobody knows anything about Lieserl Einstein-Maric. (Source)