class RtRelation
  {
  public:
    double rmin,rmax;
    double* Rt(double * x, double* par);
  } 
  RtRelation* rtrelation = new RtRelation;
  TF1* rt = new TF1("rt",rtrelation->Rt,
		    rtrelation->rmin,rtrelation->rmax,0); 
If it is possible, how should I do it ?
Regards, thanks, Wouter