More than half of all stars are found to be not in single but in binary or even multiple systems. A significant number of them will influence each other during their lifetime. This is happening mostly via mass transfer from one star to the other. This mass transfer can be stable or unstable. An unstable mass transfer leads to a common envelope around both stars resulting in a very compact binary system. For a better understanding compact binary systems, which evolved through this phase, have to be studied. About a third of the hot subdwarfs of spectral type B, which are core-helium burning stars stripped of most of their envelope on the red giant branch, are found in compact binaries with cool, low-mass stellar, substellar or white dwarf companions with periods from about one hour to a few days. The compact hot subdwarf binaries must have gone through a previous common-envelope phase. Another third of the subdwarfs are found in binaries of a few hundred days with FGK type companions. They most likely result from stable mass transfer. Investigations show that binary interaction is required for the formation of a hot subdwarf, which makes them ideal for studying mass transfer in low-mass binaries and the influence of these interactions on the future evolution of the binary system. Those compact binaries show different kinds of variations in their light curves, which can be used to characterize the primary star and its companion. Many close binaries have ground-based or space-based light curves with high signal-to-noise ratio available now, which can be used to find and analyze those systems. In this talk I will present the research we have done in the last years searching for such close binaries, as well as the plans for the future to understand the influence of close companions on late stellar evolution.