It has been stated that Prince Leopold of Hohenzollern-Sigmaringen renounced his succession rights to the throne of Romania in favor of his sons, after his brother Karl got elected as its first sovereign prince.

Later on, his own eldest son (Wilhelm) renounced his rights as heir-presumptive to his uncle (by then, King Carol I) in favor of his younger brother, Ferdinand.

Earlier in royal history, the future King Ludwig III of Bavaria renounced, when in 1868 he married Archduchess Maria Theresia of Austria-Este, his rights to the defunct throne of Greece (the house of Wittelsbach had been deposed there in 1862, but the kingdom remained, ruled by another dynasty).

My question is: were these renunciations formal, in writing? If so, where are the documents?

I suppose the same question could be raised with respect to Prince Carlo of Bourbon-Two Sicilies, who in 1900 married Infanta Maria de las Mercedes of Spain, who similarly renounced his succession rights to a defunct throne.

Unless there are written documents to prove the said renunciations, I'm not sure if they really are legally binding -- especially on future generations ...