TString rootString; t->Branch("rootString","TString",&rootString, 1600, 0);which is 'natural' as it uses the legacy syntax (branch_name,class_name, user_data) but did not work because 'rootString' is an object rather than a pointer to an object. (However the simplier form:
t->Branch("rootString",&rootString, 1600, 0);works/worked fine).