  Methyl cyanide is an important trace molecule in space, especially in
star-forming regions where it is one of the more common molecules used to
derive kinetic temperatures. We want to obtain accurate spectroscopic
parameters of minor isotopologs of methyl cyanide in their lowest excited $v_8
= 1$ vibrational states to support astronomical observations, in particular,
with interferometers such as ALMA. The laboratory rotational spectrum of methyl
cyanide in natural isotopic composition has been recorded from the millimeter
to the terahertz regions. Transitions with good signal-to-noise ratios could be
identified for the three isotopic species CH$_3^{13}$CN, $^{13}$CH$_3$CN, and
CH$_3$C(15)N up to about 1.2 THz ($J'' \le 66$). Accurate spectroscopic
parameters were obtained for all three species. The present data were already
instrumental in identifying $v_8 = 1$ lines of methyl cyanide with one $^{13}$C
in IRAM 30 m and ALMA data toward Sagittarius B2(N).
