Is it possible to an existing tree to add a new branch to another (smaller) number of entries? If so, how to do it right?
All branches in a tree are expected to have exactly the same number of entries.
Pepe is right. To do what you seem to need, we recommend creating a new TTree with the smaller number of entries and to connect the two TTree via TTree::AddFriend and TTree::BuildIndex.
Thanks for the answer.