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Context. Radial velocity (RV) measurements from near-infrared spectra havebecome a potentially powerful tool to search for planets around cool stars andsub-stellar objects. As part of a large survey to characterize M-dwarfs usingNIRSPEC at Keck II, we obtained spectra of eight late M-dwarfs (spectral typesM5.0-M8.0) during two or more observing epochs per target. These spectra weretaken with intermediate spectral resolving powers (R \sim 20,000) in theJ-band.  Aims. We search for relative radial velocity variability in these lateM-dwarfs and test the NIRSPEC capability of detecting short period brown dwarfand massive planetary companions around low-mass stars in the J-band (\approx1.25 micron). Additionally, we reanalyzed the data of the M8-type star vB10(one of our targets) presented in Zapatero Osorio et al. (2009), which wereobtained with the same instrumentation as our data.  Methods. [...]  Results. For the entire M-dwarf sample, we do not find any evidence ofrelative RV variations induced by a short period brown dwarf or massiveplanetary companion. The typical RV precision of the measurements is between180 and 300 m/s, which is sufficient to detect hot Neptunes around M-dwarfs.Also, we find that the spurious RV shift in Zapatero et al. (2009) of the starVB10 was caused by asymmetries in the instrumental profile between differentobserving epochs, which were not taken into account in their analysis.

Search for radial velocity variations in eight M-dwarfs with NIRSPEC/Keck II