We investigate the frequency of high carbon-to-oxygen ( C/O = 0.9 ) M dwarf stars in the solar neighbourhood .
Using synthetic spectra , we find that such M dwarfs would have weaker TiO bands relative to hydride features .
Similar weakening has already been detected in M-subdwarf ( sdM ) stars .
By comparing to existing spectroscopic surveys of nearby stars , we show that less than one percent of nearby stars have high carbon-to-oxygen ratios .
This limit does not include stars with C/O = 0.9 , [ m/H ] > 0.3 , and [ C/Fe ] > 0.1 , which we predict to have low-resolution optical spectra similar to solar metallicity M dwarfs .