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Emission lines indicative of active accretion have been seen on a handful oflow-mass companions (M < 30 MJup) to stars. Line variability is ubiquitous onstellar accretors but has never been characterized in detail on low-masscompanions and can give insights on the accretion mechanism at play. Weinvestigate the emission line variability of two low-mass companions (M<30MJup) to stars to understand their accretion mechanisms. Using J-bandobservations, we analyze the short to long-term variability of the HI Paschen{\beta} emission line (1.282 {\mu}m) for GQ Lup b and GSC 06214-00210 b.Archival spectroscopic observations are also examined to extend the time span.We compare their line profiles and intensities to more massive accretors andmagnetospheric accretion and shock models. Both objects have HI Paschen {\beta}flux variability that is moderate at short timescales (< 50 %) and increases atlonger timescales (~1000 % on decade timescales), resembling classical T Tauristars. GQ Lup b's line profiles are compatible with magnetospheric accretion.GSC 06214-00210 b's profiles are reproduced by both magnetospheric accretionand shock models, except for the brightest epoch for which the shock model ishighly favored. Both companions have C/O values broadly consistent with solarvalues. While magnetospheric accretion is favored for GQ Lup b, higherresolution (R > 10000) observations are required to disentangle the two(non-exclusive) line formation mechanisms. The similarity in variabilitybehavior may support similar accretion mechanisms between these low-masscompanions and classical T Tauri stars. The significant variability observed atmonths and longer timescales could explain the low yield of H{\alpha} imagingcampaigns.

Emission line variability of young 10-30 Mjup companions : I. The case of GQ Lup b and GSC 06214-00210 b