  We present the discovery of PTF 10vgv, a Type Ic supernova detected by the
Palomar Transient Factory, using the Palomar 48-inch telescope (P48). R-band
observations of the PTF 10vgv field with P48 probe the supernova emission from
its very early phases (about two weeks before R-band maximum), and set limits
on its flux in the week prior to the discovery. Our sensitive upper limits and
early detections constrain the post-shock-breakout luminosity of this event.
Via comparison to numerical (analytical) models, we derive an upper-limit of R
\lesssim 4.5 Rsun (R \lesssim 1 Rsun) on the radius of the progenitor star, a
direct indication in favor of a compact Wolf-Rayet star. Applying a similar
analysis to the historical observations of SN 1994I, yields R \lesssim 1/4 Rsun
for the progenitor radius of this supernova.
