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The progenitors of some supernovae (SNe) exhibit outbursts withsuper-Eddington luminosities prior to their final explosions. This behavior iscommon among Type IIn SNe, but the driving mechanisms of these precursors arenot yet well understood. SNHunt 275 was announced as a possible new SN duringMay 2015. Here we report on pre-explosion observations of the location of thisevent by the Palomar Transient Factory (PTF) and report the detection of aprecursor about 500 days prior to the 2015 May activity (PTF 13efv). Theobserved velocities in the 2015 transient and its 2013 precursor absorptionspectra are low (1000-2000 km/s), so it is not clear yet if the recent activityindeed marks the final disruption of the progenitor. Regardless of the natureof this event, we use the PTF photometric and spectral observations, as well asSwift-UVOT observations, to constrain the efficiency of the radiated energyrelative to the total kinetic energy of the precursor. We find that, using anorder-of-magnitude estimate and under the assumption of spherical symmetry, theratio of the radiated energy to the kinetic energy is in the range of 0.04 to3400.

PTF 13efv - An outburst 500 days prior to the SNHunt 275 explosion and its radiative efficiency