  Hot subdwarfs are considered to be the compact helium cores of red giants,
which lost almost their entire hydrogen envelope. What causes this enormous
mass loss is still unclear. Binary interactions are invoked and a significant
fraction of the hot subdwarf population is indeed found in close binaries. In a
large project we search for the close binary sdBs with the most and the least
massive companions. Significantly enhancing the known sample of close binary
sdBs we performed the first comprehensive study of this population. Triggered
by the discovery of two sdB binaries with close brown dwarf companions in the
course of this project, we were able to show that the interaction of stars with
substellar companions is an important channel to form sdB stars. Finally, we
discovered a unique and very compact binary system consisting of an sdB and a
massive white dwarf, which qualifies as progenitor candidate for a supernova
type Ia. In addition to that, we could connect those explosions to the class of
hypervelocity hot subdwarf stars, which we consider as the surviving companions
of such events. Being the stripped cores of red giants, hot subdwarfs turned
out to be important markers of peculiar events in stellar evolution ranging all
the way from star-planet interactions to the progenitors of stellar explosions
used to measure the expansion of our Universe.
