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abstract
Rotational spectra of the $^{25}$Mg and $^{26}$Mg isotopomers of MgNC have been recorded in the laboratory using mm/sub-mm direct absorption spectroscopy. Seven transitions of $^{26}$MgNC and three transitions of $^{25}$MgNC were measured in the frequency range 298-380 GHz. Rotational and fine structure parameters were determined for both species, as well as hyperfine constants for $^{25}$MgNC. These measurements have resulted in the identification of $^{25}$MgNC and $^{26}$MgNC in the circumstellar envelope of IRC+10216, and determination of the $^{24}$Mg:$^{25}$Mg:$^{26}$Mg ratio in the outer shell of this late-type star.
