Time resolved spectroscopy of luminous sdOs - searching for their massive compact companions

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Luminous hot subdwarf stars of spectral type O are the remnants of main sequence stars that lost most of their hydrogen envelopes. The formation of these stars is unclear. Recently, two of the brighest sdOs have been discovered to be X-ray binaries with massive compact companion like white dwarfs or neutron stars. Although this kind of sdO stars is known for decades, no systematic search for close companions has been conducted. We were granted seven nights at McDonald observatory to take high resolution, time resolved spectroscopy of 8 bright sdOs with the SES spectrograph mounted at the 2.1m telescope. Our goal is to measure radial velocity shifts caused by unseen, close companions and derive the orbital solutions of such binaries. The nature of the unseen companions can be constrained in this way.