  The early phases of the observed evolution of the supernovae (SNe) are
expected to be dominated by the shock breakout and ``flash" ionization of the
surrounding circumstellar medium. This material arises from the last stages of
the evolution of the progenitor, such that photometry and spectroscopy of SNe
at early times can place vital constraints on the latest and fastest
evolutionary phases leading up to stellar death. These signatures are erased by
the expansion of the ejecta within ~5 days after explosion. Here we present the
earliest constraints, to date, on the polarization of ten transients discovered
by the Zwicky Transient Facility (ZTF), between June 2018 and August 2019.
Rapid polarimetric followup was conducted using the Liverpool Telescope RINGO3
instrument, including 3 SNe observed within <1 day of detection by the ZTF. The
limits on the polarization within the first 5 days of explosion, for all SN
types, is generally <2%, implying early asymmetries are limited to axial ratios
>0.65 (assuming an oblate spheroidal configuration). We also present
polarimetric observations of the Type I Superluminous SN 2018bsz and Type II SN
2018hna, observed around and after maximum light.
