We used the OSIRIS camera at the 10.4 m Gran Telescopio Canarias ( GTC ) to monitor the astrometric motion of the L4.5 dwarf 2M1821 + 14 over 17 months .
The astrometric residuals of eleven epochs have a r.m.s .
dispersion of 0.4 mas , which is larger than the average precision of 0.23 mas per epoch and hints towards an additional signal or excess noise .
Comparison of the point-spread-functions in OSIRIS and FORS2/VLT images reveals no differences critical for high-precision astrometry , despite the GTC ’ s segmented primary mirror .
We attribute the excess noise to an unknown effect that may be uncovered with additional data .

For 2M1821 + 14 , we measured a relative parallax of 106.15 \pm 0.18 mas and determined a correction of 0.50 \pm 0.05 mas to absolute parallax , leading to a distance of 9.38 \pm 0.03 pc .
We excluded at 3- \sigma confidence the presence of a companion to 2M1821 + 14 down to a mass ratio of 0.1 ( \approx 5 M _ { \mathrm { Jupiter } } ) with a period of 50–1000 days and a separation of 0.1–0.7 au .
The accurate parallax allowed us to estimate the age and mass of 2M1821 + 14 of 120–700 Myr and 0.049 ^ { +0.014 } _ { -0.024 } M _ { \odot } , thus confirming its intermediate age and substellar mass .
We complement our study with a parallax and proper motion catalogue of 587 stars ( i ^ { \prime } \simeq 15.5 - 22 ) close to 2M1821 + 14 , used as astrometric references .
This study demonstrates sub-mas astrometry with the GTC , a capability applicable for a variety of science cases including the search for extrasolar planets and relevant for future astrometric observations with E-ELT and TMT .