Double_t mydgauss(Double_t *x, Double_t *par) {
 if (par[2] != 0 && par[3] != 0)  {
   Double_t arg1 =(x[0]-par[1])/par[2];
   Double_t arg2 =(x[0]-par[1])/par[3];
   Double_t r1 = TMath::Exp(-arg1*arg1);
   Double_t r2 = TMath::Exp(-arg2*arg2);
   return par[0]*(r1+r2);
 }
}


void doufit()
{
  float x[16]={2.5,2.59,2.53,2.56,2.62,2.65,2.47,2.44,2.41,2.38,2.35,2.32,2.23,2.17,2.11,1.99};
  float y[16]={7.51e-9,3.81e-10,4.47e-9,3.12e-9,4.91e-11,7.62e-12,3.25e-8,4.19e-8,3.28e-8,2.89e-8,2.97e-8,2.19e-8,1.22e-8,4.83e-9,1.36e-9,1.9e-10};
  TGraph *g=new TGraph(16,x,y);
  TF1 *gg=new TF1("gg","gaus",2.3,2.7);
  g->Fit("gg");
  TF1 *f1=new TF1("f1",mydgauss,1.9,2.7,4);
  double par[4];
  par[0]=gg->GetParameter(0);
  par[1]=gg->GetParameter(1);
  par[2]=10*gg->GetParameter(2);
  par[3]=0.1*gg->GetParameter(2);
  f1->SetParameters(par);
  g->Fit(f1);
  g->Draw("A*");
}