  The nature and role of the binary companion of carbon-oxygen white dwarf
stars that explode as Type Ia supernovae (SNe Ia) are not yet fully understood.
Past detections of circumstellar material (CSM) that contain hydrogen for a
small number of SN Ia progenitor systems suggest that at least some have a
nondegenerate companion. In order to constrain the prevalence, location, and
quantity of CSM in SN Ia systems, we performed a near-ultraviolet (NUV) survey
with the Hubble Space Telescope (HST) to look for the high-energy signature of
SN Ia ejecta interacting with CSM. Our survey revealed that SN 2015cp, a SN
1991T-like overluminous SN Ia, was experiencing late-onset interaction between
its ejecta and surrounding CSM at $664$ days after its light-curve peak. We
present ground- and space-based follow-up observations of SN 2015cp that reveal
optical emission lines of H and Ca, typical signatures of ejecta-CSM
interaction. We show how SN 2015cp was likely similar to the well-studied SN
Ia-CSM event PTF11kx, making it the second case in which an unambiguously
classified SN Ia was observed to interact with a distant shell of CSM that
contains hydrogen ($R_{\rm CSM} \gtrsim 10^{16}\ {\rm cm}$). The remainder of
our HST NUV images of SNe Ia were nondetections that we use to constrain the
occurrence rate of observable late-onset CSM interaction. We apply theoretical
models for the emission from ejecta-CSM interaction to our NUV nondetections,
and place upper limits on the mass and radial extent of CSM in SN Ia progenitor
systems.
