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tokens: Wyatt ( 2014 ) showed that the disc should be truncated at a semimajor axis (at apocentre) where R H,Q is the Hill radius of an eccentric planet at apocentre. Under the influence of two planets, that inner edge could be further out if the strongest secular resonance is located beyond a in . Using equation (24) in Yelverton Kennedy ( 2018 ), we find the approximate location of the resonance to be (dashed blue line in Fig. 9 ) where a 1 and a 2 are the semimajor axes of the companions, while m 1 and m 2 are their masses, with 1 designating the innermost companion and 2 the BD. In order to effectively truncate the disc, the width of the secular resonance would need to be large enough to excite solids from a in to . This width is controlled primarily by the eccentricity of HD 206893 B. Given the uncertainties on its eccentricity () and its mass (12–50 M Jup ; Delorme et al. 2017 ), it is hard to assess whether secular resonances will deplete a region that otherwise would be stable. Assuming the best-fitting values of our fit and using equation (36) in Yelverton Kennedy ( 2018 ), we expect a secular resonance width of ∼20 au. Therefore, it is possible that the disc inner edge is not truncated by the BD, but by secular resonances. Note that HD 206893 B alone could truncate the disc out to ∼30 au if it has an eccentricity of 0.2 and a mass of 50 M Jup . A more precise characterization of HD 206893 B’s orbit and the disc inner edge could provide better estimates of the masses of the two inner companions. 5.2.3 Secular resonances between BD and planet at 30 au Secular resonances might still have created the gap if instead the resonance at 74 au was with the BD at 11 au and an outer planet sitting just interior to the disc inner edge at around 30 au. We find that this putative planet and the BD at 11 au would need to have a mass ratio close to unity (see solid orange line in Fig. 9 ). This is ruled out by direct imaging, which did not detect any companion more massive than 3 M Jup at 30 au (assuming an age of 50 Myr; Delorme et al. 2017 ). 5.2.4 Secular resonances between BD and planet at 74 au If the gap at 74 au was indeed carved by a planet at that distance, it is worth discussing what other observables could confirm this scenario. Given the range of possible masses of the BD and the putative planet c at 74 au, a secular resonance could be located in between the orbit of b and the disc outer edge. We find that the resonance would be located between 85 and 170 au for mass ratios of b and B of 10 −3 to 0.3 (solid green line in Fig. 9 ). This resonance would create a gap that is expected to be very wide (≳50 au if b has an eccentricity of ≳0.05 according to equation 36 in Yelverton Kennedy 2018 ), and thus noticeable by our observations. This means that we can already rule out that this gap is present in between 74 and 110 au, otherwise the gap would be seen to be much wider and the disc peak further out (or at 40 au instead). Therefore, imposing