There is a long rumour that Crown Prince Rudolf fell for his cousin Archduchess Maria Antonia, the only daughter of Grand Duke Ferdinand IV of Tuscany and his first wife, a princess from the Royal House of Saxony.
When her mother died, Maria Antonia was still quite small. Although her father remarried later they were still very close. However Maria Antonia inherited her mother's frail health. When Crown Prince Rudolf met his cousin, he fell deeply in love with this lovely, quiet and talented princess.
When he asked for permission to marry her, Emperor Franz Josef refused, one of the reason being that she she was ill and not likely to have a long life (most likely he thought back on his own sick wife, Rudolf's mother Sisi).
However things got sticky when Maria Antonia discovered that she was with child. So Rudolf married Maria Antonia in secret and the child was later given up to adoption. Maria Antonia never got over the shock, and decided to enter into holy orders. She died soon after Rudolf married.