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tokens: peak value and rms noise. Both weighting methods provide comparable (12 per cent difference) integrated flux densities. We followed the same strategy to obtain the image of the line emission, by cleaning with different velocity resolutions up to 6.3 km s −1 . The H30 α recombination line was not detected in the data cube, using either Briggs or natural weighting. We were able to clean each velocity channel to 0.1 mJy beam −1 (rms), which may be taken as an upper limit to the line flux density. The recombination line flux estimated in local thermodynamic equilibrium using the emission measure from Section 3 falls below our detection limit. 3 RESULTS The continuum emission of the shell at 230 GHz is shown in Fig. 1a . This is the best resolution image calculated from our data. Fig. 1b displays the best resolution image contours superposed to the natural weighting, lower resolution map, which confirms the structures seen in the best resolution image. An even lower resolution image, aimed to match the Hubble Space Telescope ( HST ) point spread function at H α, simulates the expected emission of an optically thin recombination line ( Fig. 1c ). This image, derived directly from the high resolution image ( Fig. 1a ) using a Gaussian kernel convolution, is remarkably similar to the ring/bipolar shell morphology proposed by Harvey et al. ( 2018 ) for this nova. Most, if not all the large-scale structures seen in Fig. 1c is due to the blending of a distribution of smaller unresolved sources. Large contiguous emission regions that could be identified as ring or bipolar caps, are absent above the 2.5σ level in ALMA high resolution image, although they may be present at fainter flux densities. The total measured shell flux density is 9.4 ± 1.5 mJy. Figure 1. ALMA image of the V5668 Sgr shell at 230 GHz. The top panel (a) shows the full resolution continuum image above 3.0σ. Major tick marks are spaced by 100 mas. The middle panel (b) shows the natural weighted image with five superposed contour levels from the image in panel (a), starting at 2.5σ. The bottom panel (c) displays the image given in panel (a) after a Gaussian convolution to match the HST spatial resolution at H α. The flux unit is Jy beam −1 for panels (a) and (b), and arbitrary units for panel (c). North is at the top and east to the left. The power spectrum of the continuum image ( Fig. 2 ) shows that a significant fraction of the emission is formed in spatially resolved structures. It also indicates that features above the noise level are present at scales as small as 55 mas or 1 × 10 15 cm, i.e. near the resolution limit of the observations. A distance of 1540 pc is assumed in this work (Banerjee et al. 2016 ), a value that is in agreement with both the observed shell expansion parallax for V exp = 650 km s −1 and maximum magnitude and rate of decay relations. No parallax to this object could be found in Gaia DR2. Measurement of individual clump sizes in the image are consistent with