After the master's death, a letter to the Immortal Beloved was found in a hidden drawer of the desk (found together with, among others, the Heiligenstadt Testament).

Unfortunately, it was not possible to determine who the mysterious addressee of the letter was, nor is it known where the letter was addressed to, when it was written, and whether the author sent it at all. There are speculations that this beloved was Dorothea von Ertmann.