Suppose I have a programme. In there I have the creation of histograms: signal: histo_s_Pt histo_s_v1 histo_s_v2 background: histo_b_Pt, histo_b_v1 histo_b_v2 iteration to fill signal/bckg from events j. So at this point I'm having the 6 above histograms filled. Now if I wanted to compare the variables v1,v2 between signal/background with the reweight of the momenta Pt s or b, I have to make a new histogram that will contain the weights : histo_weight = Divide( signal Pt , background Pt). My problem is that this can happen only at the end of the program, and then I cannot call these weights back in order to reweight the histos of variables v1, v2 . I was told I can save the histo_weight in some root file and then recall it. However I am not sure that this can work. Suppose I save it in a TFile weight.root How can I open the weight.root in the first place? since it again is saved in the end. Any ideas?