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In the second paper of this series we perfected our method of linking highprecision Hubble Space Telescope astrometry to the high-accuracy Gaia DR2absolute reference system to overcome the limitations of relative astrometrywith narrow-field cameras. Our test case here is the Y brown dwarf WISEJ163940.83-684738.6, observed at different epochs spread over a 6-yr timebaseline with the Infra-Red channel of the Wide Field Camera 3. We derivedsignificantly improved astrometric parameters compared to previousdeterminations, finding: (mu_RAcosDc,mu_DC,parallax) =(577.21+/-0.24mas/yr,-3108.39+/-0.27mas/yr,210.4+/-1.8mas). In particular, ourderived absolute parallax corresponds to a distance of 4.75+/-0.05pc for thefaint ultracool dwarf.

Extending Gaia DR2 with HST narrow-field astrometry. II. Refining the method on WISE J163940.83-684738.6