Instantaneous RV of star 3 with respect to system center-of-mass

def RV_3(self):
        """Instantaneous RV of star 3 with respect to system center-of-mass
        """
        return -self.orbpop_long.RV * (self.orbpop_long.M1 / (self.orbpop_long.M1 + self.orbpop_long.M2)) +\
            self.orbpop_short.RV_com2