tokens: L5 dwarfs includes 10 L4.5, 24 L5, and 2 L5.5 sources with seven detections of H in emission, and three classified as variable ( Schmidt et al. 2007 , 2015 ). In their sample, Pineda et al. (2016) included three L5 with H in emission in one of them. They inferred a fraction of emitters of 20% (15–35%; 68% confidence limit), consistent with the interval from the SDSS sample ( Schmidt et al. 2007 , 2015 ) although to date there are few sources. We also note that a large number of L dwarfs show variability in their level of chromospheric activity ( Liebert et al. 2003 ; Reiners Basri 2008 ; Schmidt et al. 2015 ; Pineda et al. 2016 ), the most striking case being the L5.5eT7 binary 2MASSI J1315309264951AB (L5.5; Hall 2002a ; Gizis 2002 ; Burgasser et al. 2011 , 2013 , 2015 ). 4.3. Radial velocity and space motion We measured the position of the H line in the 1D spectrum with the splot task under IRAF. We identified the line at 6564.11 Å with an error of 0.04 Å, while the nominal air wavelength from the NIST Atomic Spectra Database is 6562.819 Å. We should add in quadrature the error on our wavelength calibration (2.3 km s, see Sect. 3.2 ) and uncertainty due to the timespan in our observations (0.2 km s). We inferred a radial velocity of 38.0 2.9 km s (Table 2 ) after correcting for the rotation of the Earth, the motion of the Earth around the Earth-Moon barycentre, the orbit of the barycentre about the Sun, and motion of the Sun relative to the specified standard of rest. Our value is in close agreement with the mean values derived independently from the ground (39.1 0.2 km s; Detweiler et al. 1984 ) and from space with H ipparcos (39.48 0.30 km s; de Bruijne et al. 2001 ). We infer a space motion of [] = [42.92, 23.96, 0.72] km s for 2M041821 (Table 2 ), in close agreement with the space motion of the cluster centre: [41.1, 19.2, 1.4] km s ( van Leeuwen 2009 ). According to Vizier, 2M041821 should have a Gaia -band of 19.096 and should be included in the Initial Gaia Source List ( Smart Nicastro 2014 ), implying that precise coordinates and proper motions will be soon available since the second Gaia data release is expected at the end of 2017. However, based on the equation 0.244 SpT 12.6332 kindly provided by R. Smart, 2M041821 might have 20.4 mag, placing it at the faint end of the Gaia catalogue. Therefore, it is unclear at this stage whether its membership to the Hyades cluster will be settled by Gaia in the near future. Table 2 Results of our spectroscopic analysis. Notes. We quote the spectral type, spectroscopic distance, radial velocity, space motion, and ratio of the (/) luminosity for 2M041821. 4.4. Lithium absorption The lithium test was proposed in the early 1990s to distinguish between very low-mass stars and brown dwarfs ( Rebolo et al. 1992 ; Magazzu et al. 1993 ). The technique relies on the detection of lithium in absorption at 6707.8 Å for late-M and L dwarfs straddling the stellar-substellar boundary. If lithium is detected,
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