These sources are known as HW Vir kind of systems and believed to construct a high percentage of subdwarfs despite not proven yet. The aim of this project was to look at the APOGEE spectra of late-type stars in order to find a trace of an sdB companion. Obtaining a high-quality IR observation for such systems would let us learn the physical parameters of the low-mass companion. Despite there are many scenarios to form both single and binary subdwarf stars, binarity in these stars is not well understood. Additionally, there are theories being discussed in the favour of the idea that subdwarfs are essentially a component of binary systems. Yet, the properties of the browndwarf components are mostly unknown, this keeps the formation and evolution of sdBs as a mystery. As the project went on, it evolved in a way to focus on HW Vir system itself. When the APOGEE data quality was not satisfactory to resolve the spectra of components or to improve the current knowledge of the physical parameters, we observed the system with IGRINS in February 2016 and in April 2016. At the final stage of the project, I worked on finding the appropriate models and the most effective approach to apply the model spectra. The solution of the system may help us to improve the parameter set of the cool component and also contribute to revealing the binary nature of sdB stars and common-envelope evolution.

The aim of my project is to examine binary systems containing at least one unusual hot subdwarf. These systems allow us to explore possible outcomes of common- envelope (CE) ejection in a close binary. Depending on the observational results (i.e. orbital period, mass ratio, chemical abundances, etc.) a conclusion for these exotic systems can be reached regarding an idea of the formation and evolution of He-rich subdwarf binaries, especially regarding the CE evolution. Besides a general perspective on their formation, they will also help us to go further on stellar evolution and confirm our current knowledge about the outcomes of CE evolution. The first object studied was PG 1544+488, an exceptional short-period spectroscopic binary containing two helium-rich subdwarfs. I obtained and improved orbital and atmospheric parameters for each component. We concluded that in a CE binary containing two helium cores of nearly equal mass, the entire hydrogen envelopes of both stars are ejected. This places a very strong constraint on the physics of the CE ejection mechanism. The second object was BILyn and the first goal was to determine whether the relative motion of the two components could be resolved, and subsequently to obtain the dynamical mass ratio. Radial velocities of both components were measured but it was not possible to determine a period. An emission feature at Hα is also observed, which varies within a single night’s observation. The origin of this feature is discussed. Additionally, 17 subdwarf stars, are classified according to their helium abundances. For two of these; I obtained and improved orbital and atmospheric parameters for [CW83]1419-09 and recognised a variability of radial velocity for CD−31◦4800. Exotic binaries like PG1544+488 and BILyn place strong constraints on models for binary-star evolution. Further observations are required for BI Lyn, and to discover other exotic post-common- envelope binaries.