  Substructures in disc density are ubiquitous in the bright extended discs
that are observed with high resolution. These substructures are intimately
linked to the physical mechanisms driving planet formation and disc evolution.
Surveys of star-forming regions find that most discs are in fact compact, less
luminous, and do not exhibit these same substructures. It remains unclear
whether compact discs also have similar substructures or if they are
featureless. This suggests that different planet formation and disc evolution
mechanisms operate in these discs. We investigated evidence of substructure
within two compact discs around the stars Sz 65 and Sz 66 using high angular
resolution observations with ALMA at 1.3 mm. The two stars form a wide-binary
system with 6.36 arcsec separation. The continuum observations achieve a
synthesised beam major axis of 0.026 arcsec, equivalent to about 4.0 au,
enabling a search for substructure on these spatial scales and a
characterisation of the gas and dust disc sizes with high precision. We
analysed the data in the image plane through an analysis of reconstructed
images, as well as in the uv plane by modelling the visibilities and by an
analysis of the 12CO emission line. Comparisons were made with high-resolution
observations of compact discs and radially extended discs. We find evidence of
substructure in the dust distribution of Sz 65, namely a shallow gap centred at
approximately 20 au, with an emission ring exterior to it. Ninety percent of
the measured continuum flux is found within 27 au, and the distance for 12CO is
142 au. The observations show that Sz 66 is very compact: 90 per cent of the
continuum flux is contained within 16 au, and 48 au for the gas. While the
overall prevalence and diversity of substructure in compact discs relative to
larger discs is yet to be determined, we find evidence that substructures can
exist in compact discs.
