docs: 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