Iran's Supreme Leader: Who might succeed Ali Khamenei?

Recent rumours about the health of Iran's Supreme Leader, Ayatollah Ali Khamenei, have thrown the spotlight on what will happen if he becomes too ill to rule, or dies.

The 81-year-old is the highest political authority in one of the most powerful countries in the Middle East, and who will succeed him matters greatly to Iran, the region and the rest of the world.