Instantaneous RV of star 1 with respect to system center-of-mass

def RV_1(self):
        """Instantaneous RV of star 1 with respect to system center-of-mass
        """
        return self.orbpop_long.RV * (self.orbpop_long.M2 / (self.orbpop_long.M1 + self.orbpop_long.M2))