Context : High contrast imaging enables the determination of orbital parameters for substellar companions ( planets , brown dwarfs ) from the observed relative astrometry and the estimation of model and age-dependent masses from their observed magnitudes or spectra .
Combining astrometric positions with radial velocity gives direct constraints on the orbit and on the dynamical masses of companions .
A brown dwarf was discovered with the VLT/S PHERE instrument at the Very Large Telescope ( VLT ) in 2017 , which orbits at \sim \SI { 11 } { au } around HD 206893 .
Its mass was estimated between 12 and \SI { 50 } { M _ { J } } from evolutionary models and its photometry .
However , given the significant uncertainty on the age of the system and the peculiar spectrophotometric properties of the companion , this mass is not well constrained .

Aims : We aim at constraining the orbit and dynamical mass of HD 206893 B .

Methods : We combined radial velocity data obtained with H ARPS spectra and astrometric data obtained with the high contrast imaging VLT/S PHERE and VLT/N aCo instruments , with a time baseline less than three years .
We then combined those data with astrometry data obtained by Hipparcos and Gaia with a time baseline of 24 years .
We used a Markov chain Monte Carlo approach to estimate the orbital parameters and dynamical mass of the brown dwarf from those data .

Results : We infer a period between 21 and \SI { 33 } { yr } and an inclination in the range 20 - \SI { 41 } { \degree } from pole-on from HD 206893 B relative astrometry .
The RV data show a significant RV drift over \SI { 1.6 } { yr } .
We show that HD 206893 B can not be the source of this observed RV drift as it would lead to a dynamical mass inconsistent with its photometry and spectra and with Hipparcos and Gaia data .
An additional inner ( semimajor axis in the range 1.4 - 2.6 \si { au } ) and massive ( \sim \SI { 15 } { M _ { J } } ) companion is needed to explain the RV drift , which is compatible with the available astrometric data of the star , as well as with the VLT/S PHERE and VLT/N aCo nondetection .

Conclusions :