code: def dRV(self, dt, band='g'):
          
          return (self.orbpop.dRV_1(dt)*self.A_brighter(band) +
                  self.orbpop.dRV_2(dt)*self.BC_brighter(band))
docs: Returns dRV of star A, if A is brighter than B+C, or of star B if B+C is brighter