What is Rotational constant, isotopologue?

The rotational constant of a linear molecule is $B = h/(8\pi^2cI)$, in $\mathrm{cm}^{-1}$ (or $B = h/8\pi^2I$ in hertz), so that its rotational levels are $F(J) = BJ(J+1)$ in wavenumbers. Molecules that differ only in the isotopes of their atoms are isotopologues ($\ce{^{12}C^{16}O}$, $\ce{^{13}C^{16}O}$); in the Born–Oppenheimer approximation they share the same bond lengths and force constants.