While modern surveys are able to discover and classify new optical transients within
one day or less from their explosions, the proper characterization of their progenitor
stars is still a partially unsolved astrophysical issue.
The most common interacting supernovae are believed to arise from the core-collapse of
massive stars ejecting fractions of their envelopes during the last stages of their lives.
Direct evidences of luminous blue variables as progenitors of these transients is enclosed
in deep archival images, while several indirect evidences point to less massive stars as
candidate progenitors of at least a fraction of these transients.
Indirect methods can help us to determine physical properties of progenitors and their
surroundings, and require high-quality data, and good multi-wavelength monitoring of
interacting transients up to very late times after explosion.
During this colloquium, we will see how long-term photometric and spectroscopic follow-up
campaigns, combined with early discoveries and multi-wavelength observations, can help
us in characterizing this relatively rare and still poorly understood class of optical transients.