The accurate evolutionary models of substellar objects are essential to understand their atmospheric physics or predicting masses for newly found planetary-mass companions. We can validate them by measuring the characteristics of the companion brown dwarfs (BDs) independent of evolutionary models. While most of them have masses consistent with predictions from models, there are several exceptions. Gliese 229B (Gl 229B) is a well-known target for clearly showing the discrepancy with theoretical models (Brandt et al. 2020), which might suggest that there is an unseen companion. In this proposal, we aim to search for a companion around companion BDs with IRD+AO188 by using the radial velocity (RV) method. The preliminary result from past observations shows that the RV precision is sufficient to detect planetary-mass companions around BDs. Testing the multiplicity of benchmark BDs will have a great impact on existing theories. Also, these observations are fundamental for future surveys of low-mass companions around BDs. We need to both spatially resolve a BD from its nearby host star and obtain high-resolution spectra in the near-infrared, which can only be achieved with the combination of IRD and AO188.
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