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We develop a method to identify the spectroscopic signature of unresolvedL-dwarf ultracool companions, which compares the spectra of candidates andtheir associated control stars using spectral ratio differences and residualspectra. We present SpeX prism-mode spectra (0.7-2.5 micron) for a pilot sampleof 111 mid M dwarfs, including 28 that were previously identified as candidatesfor unresolved ultracool companionship (a sub-sample from Cook et al. 2016;paper 1) and 83 single M dwarfs that were optically colour-similar to thesecandidates (which we use as `control stars'). We identify four candidates withevidence for near-infrared excess. One of these (WISE J100202.50+074136.3)shows strong evidence for an unresolved L dwarf companion in both its spectralratio difference and its residual spectra, two most likely have a differentsource for the near-infrared excess, and the other may be due to spectralnoise. We also establish expectations for a null result (i.e. by searching forcompanionship signatures around the M dwarf control stars), as well asdetermining the expected outcome for ubiquitous companionship (as a means ofcomparison with our actual results), using artificially generated unresolvedM+L dwarf spectra. The results of these analyses are compared to those for thecandidate sample, and reasonable consistency is found. With a full follow-upprogramme of our candidates sample from Cook et al., we might expect to confirmup to 40 such companions in the future, adding extensively to the known desertpopulation of M3-M5 dwarfs.

Low-resolution near-infrared spectroscopic signatures of unresolved ultracool companions to M dwarfs