tokens: fitted separately. Posterior distributions of { M disc , γ, R c , h c , ψ} are presented in Figs 2 and 3 , while the best-fitting models are shown in Fig. 4 . Best-fit parameters are listed in Table 2 . Figure 2. Posterior distributions of each disc parameter, including their marginalized distributions for V2775 Ori, V1647 Ori, and NY Ori. The vertical dashed lines represent the 16th, 50th, and 84th percentiles. Contours correspond to 68 per cent, 95 per cent, and 99.7 per cent confidence regions. These plots were produced using the python module corner (Foreman-Mackey et al. 2013 ). Figure 3. Same as Fig. 2 , but for V883 Ori and HBC 494. Figure 4. The ALMA 1.3 mm continuum data (left), synthetic best-fitting models (middle), and the residuals (right) for the five resolved discs in our sample. Table 2. Disc parameters from radiative transfer modelling. 4.2.1 Radiative transfer results We find that, at the observed resolution of 0.2 arcsec, V2775 Ori, V1647 Ori, and NY Ori are well described by our parametric models: the posterior distributions of the disc parameters are relatively narrow and single peaked (see Fig. 2 ). However, V883 Ori and HBC 494 have disc parameters that are less well defined. In the case of V883 Ori, the posterior distributions for M disc , R c , and Ψ are double peaked (see Fig. 2 , left-hand panel). We speculate that this is due to the particular structure of V883 Ori, showing a very bright core, 0.2 arcsec in diameter, surrounded by a much more tenuous outer disc. The boundary between these two regions has been interpreted as the water snowline, resulting in a discontinuity in the disc properties (Cieza et al. 2016 ). The water snowline is typically at ∼3–5 au for solar-mass stars and would be unidentifiable at the 80 au resolution of our observations, but given the extreme accretion rate of V883 Ori, it has been displaced to a radius of ∼40 au. Detailed modelling of the V883 Ori disc at ∼0.03 arcsec (15 au) resolution will be presented in a follow-up paper. Similarly, in the case of HBC 494, the posterior distributions for M disc and R c are wider than for the other sources (see Fig. 3 , right-hand panel). This is likely due to the fact that the disc is not radially symmetric, preventing the MCMC exploration of parameters to converge to a narrower range of values. The disc asymmetry in HBC 494 can be seen in Figs 1 and 4 as an elongation of the disc on the south-east direction. We speculate that this asymmetry might indicate the presence of two barely resolved discs (e.g. the system might be a binary as V1118 Ori and FUor itself; Hales et al. 2015 ; Liu et al. 2017 ) or a non-axisymmetric disc structure. Fig. 5 shows a ∼8 mJy residual in our ALMA image after subtracting the best-fitting model, but in different scaling than Fig. 4 . This residual has the same intensity level ( ∼ 7 per cent of the total flux) as in other sources, but is compact and not radially symmetric. A particularly interesting possibility for this
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