The circumbinary disc of HD 34700A

Context. ALMA observations have shown that substructures are ubiquitous in protoplanetary discs. A sub-group, the transition discs, shows large cavities and rings in the dust continuum. Among these, some present very high contrast asymmetries possibly due to the presence of vortices. HD 34700A is a binary system featuring a cavity, a ring, and multiple spiral arms detected in scattered light, a prominent crescent in the ALMA continuum, and a complex gas morphology possibly connected with ongoing infall. Aims. We present new ALMA band 6 (1.3 mm) continuum images of the circumbinary disc around HD 34700A and compare them with two other systems showcasing high (greater than or similar to 30, measured as the peak-to-azimuthal-average ratio) contrast continuum asymmetries, IRS 48 and HD 142527. We aim to characterise the crescent morphology, discuss their possible origin, and, in the case of the vortex scenario, assess the efficiency of dust trapping in these systems. Methods. We performed visibility modelling of the new high-resolution (0 ''.11 x 0 ''.09) ALMA band 6 continuum data of HD 34700A, together with improved visibility modelling of the other two targets. Results. We detected a 0 ''.46 (161 au) large cavity and resolved a ring with an asymmetric crescent and an extended tail at 0 ''.53 (186 au) with a peak intensity of 1.9 mJybeam(-1), corresponding to the second highest contrast (similar to 62) ever detected with ALMA in a protoplanetary disc. We also detected unresolved emission inside the cavity, which we attribute to an inner disc. Our visibility model is in remarkable agreement with the HD 34700A data, featuring only localised residuals in the region of the disc corresponding to the tail of the asymmetry. For HD 142527, we obtained very good overall agreement with the data, recovering both the double peaked asymmetric ring and the inner disc emission. In the case of IRS 48, we recovered the general morphology of the asymmetry, but we could not reproduce the fainter ring. We then ran a hydrodynamic model of a vortex with different dust fluids, reproducing the general morphology observed in the HD 34700A and IRS 48 systems, with the emission around the vortex showing a mild asymmetry between the leading and trailing sides. Conclusions. With a combination of visibility, dust evolution, and hydrodynamical models, we have constrained the morphology of the dust continuum emission of HD 34700A for the first time, and improved existing models for IRS 48 and HD 142527. The high azimuthal contrast of the asymmetries rules out the orbit clustering of eccentric cavities scenario, while the dust evolution models we consider suggest that the vortex scenario is a plausible option.