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High cadence transient surveys are able to capture supernovae closer to theirfirst light than before. Applying analytical models to such early emission, wecan constrain the progenitor stars properties. In this paper, we presentobservations of SN2018fif (ZTF18abokyfk). The supernova was discovered close tofirst light and monitored by the Zwicky Transient Facility (ZTF) and the NeilGehrels Swift Observatory. Early spectroscopic observations suggest that theprogenitor of SN2018fif was surrounded by relatively small amounts ofcircumstellar material (CSM) compared to all previous cases. Thisparticularity, coupled with the high cadence multiple-band coverage, makes it agood candidate to investigate using shock-cooling models. We employ theSOPRANOS code, an implementation of the model by Sapir & Waxman and itsextension to early times by Morag, Sapir & Waxman. Compared with previousimplementations, SOPRANOS has the advantage of including a careful account ofthe limited temporal validity domain of the shock-cooling model as well asallowing usage of the entirety of the early UV data. We find that theprogenitor of SN2018fif was a large red supergiant, with a radius ofR=744.0_{-128.0}^{+183.0} solar radii and an ejected mass ofMej=9.3_{-5.8}^{+0.4} solar masses. Our model also gives information on theexplosion epoch, the progenitor inner structure, the shock velocity and theextinction. The distribution of radii is double-peaked, with lower radiicorresponding to lower values of the extinction, earlier recombination timesand better match to the early UV data. If these correlations persist in futureobjects, denser spectroscopic monitoring constraining the time ofrecombination, as well as accurate UV observations (e.g. with ULTRASAT), willhelp break the radius-extinction degeneracy and independently determine both.

SN 2018fif: The Explosion of a Large Red Supergiant Discovered in Its Infancy by the Zwicky Transient Facility