Rotational spectroscopy of isotopic oxirane, $c$-C$_2$H$_4$O
  We studied the rotational spectrum of oxirane in a sample of natural isotopic
composition in selected regions between 158 GHz and 1093 GHz. Investigations of
the isotopologs with one $^{13}$C or one $^{18}$O were the primary focus in
order to facilitate searches for them in space. We also examined the main
isotopic species mainly to look into the performance of Watson's A and S
reductions both in an oblate and in a prolate representation. Even though
oxirane is a rather asymmetric oblate rotor, the A reduction in the III$^l$
representation did not yield a satisfactory fit, as was observed frequently
earlier for other molecules. The other three combinations yielded satisfactory
fits of similar quality among each other; the A reduction in the I$^r$
representation required two parameters less than both S reduction fits.
