  We present multi-wavelength observations of SN2014C during the first 500
days. These observations represent the first solid detection of a young
extragalactic stripped-envelope SN out to high-energy X-rays. SN2014C was the
explosion of an H-stripped progenitor star with ordinary explosion parameters.
However, over the time scale of ~1yr, SN2014C experienced a complete
metamorphosis and evolved from an ordinary H-poor supernova of type Ib into a
strongly interacting, H-rich supernova of type IIn. Signatures of the SN shock
interacting with a dense medium are observed across the spectrum. Coordinated
observations with Swift, Chandra and NuSTAR have captured the evolution in
detail and revealed the presence of a massive shell of ~1 Msun of hydrogen-rich
material at ~6d16 cm from the explosion site. We estimate that the shell was
ejected by the progenitor star in the decades to centuries before core
collapse. This result poses significant challenges to current theories of
massive star evolution, as it requires a physical mechanism responsible for the
ejection of the deepest hydrogen layer of H-poor SN progenitors synchronized
with the onset of stellar collapse. Theoretical investigations point at binary
interactions and/or instabilities during the last stages of nuclear burning in
massive stars as potential triggers of the time-dependent mass loss. We
constrain these scenarios utilizing the sample of 183 SNe Ib/c with public
radio observations. Our analysis identifies SN2014C-like signatures in ~10% of
SNe with constraining radio data. This fraction is somewhat larger but
reasonably consistent with the expectation from the theory of recent envelope
ejection due to binary evolution IF the ejected material can survive in the
close environment for 1000-10000 yrs. Alternatively, nuclear burning
instabilities extending all the way to the core C-burning phase might also play
a critical role.
