void bla() {


  // -- The new file with the custom tree to be written
  TFile *n = new TFile("new.root", "RECREATE"); 
  TTree* tt = new TTree("tt","tt");

  // -- The variables you want in the new tree
  double npcms, nbla;
  tt->Branch("pcms", &npcms ,"pcms/D");
  tt->Branch("bla",&nbla,"bla/D");
 
  // -- The file with the big tree
  TFile *o = new TFile("old.root"); 
  TTree *t = (TTree*)o->Get("oldtree");

  // -- Get the needed variables from the old tree
  double bmxhad, bpcms;
  t->SetBranchAddress("mxhad",&bmxhad);
  t->SetBranchAddress("pcms",&bpcms);

  double bla;

  // -- Read in old tree
  int nb, nbytes; 
  Int_t nentries = Int_t(t->GetEntries());
  for (Int_t jentry = 0; jentry < nentries; jentry++) {
    nb = t->GetEntry(jentry);   nbytes += nb;

    bla = 4.123*bpcms; // just to compute a new variable ...

    npcms = bpcms; 
    nbla = bla; 

    tt->Fill(); 
  }

  n->Write(); 
}