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The binary fraction in the sub-stellar regime is a topic of discussion. The lower masses of ultra cool dwarfs (UCDs) with respect to the other stars make them even more important because a measurable effect on their radial velocity (RV) or luminosities can be caused by extremely low mass companions. Some UCDs in young star forming regions are bright enough to be studied with existing high resolution instrumentation. The UCDs are intrinsically faint in the optical and the optical RV measurements are affected by “rotationally modulated inhomogeneous surface features” that can mimic a companion, while the near-infrared (NIR) RVs are less prone to them. Therefore, we decided to monitor the RV of six UCDs in the NIR. Blake et al. (2007) demonstrated RV measurement accuracy of 300-600 m s−1 in the NIR using telluric calibration. [ABSTRACT FROM PUBLISHER]