  We present new Hubble Space Telescope (HST) multi-epoch ultraviolet (UV)
spectra of the bright Type IIb SN 2013df, and undertake a comprehensive anal-
ysis of the set of four Type IIb supernovae for which HST UV spectra are
available (SN 1993J, SN 2001ig, SN 2011dh, and SN 2013df). We find strong
diversity in both continuum levels and line features among these objects. We
use radiative-transfer models that fit the optical part of the spectrum well,
and find that in three of these four events we see a UV continuum flux excess,
apparently unaffected by line absorption. We hypothesize that this emission
originates above the photosphere, and is related to interaction with
circumstel- lar material (CSM) located in close proximity to the SN progenitor.
In contrast, the spectra of SN 2001ig are well fit by single-temperature
models, display weak continuum and strong reverse-fluorescence features, and
are similar to spectra of radioactive 56Ni-dominated Type Ia supernovae. A
comparison of the early shock-cooling components in the observed light curves
with the UV continuum levels which we assume trace the strength of CSM
interaction suggests that events with slower cooling have stronger CSM
emission. The radio emission from events having a prominent UV excess is
perhaps consistent with slower blast-wave velocities, as expected if the
explosion shock was slowed down by the CSM that is also responsible for the
strong UV, but this connection is currently speculative as it is based on only
a few events.
