Nobel's third and long-lasting love was with a flower girl named Sofie Hess from Vienna. This liaison lasted for 18 years and in many of the exchanged letters, Nobel addressed his love as 'Madame Sofie Nobel'. After his death, according to his biographers – Evlanoff and Fluor, and Fant – Nobel's letters were locked within the Nobel Institute in Stockholm and became the best-kept secret of the time.