Instantaneous RV of star 2 with respect to system center-of-mass

def RV_2(self):
        """Instantaneous RV of star 2 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_com1