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The origin of hot subdwarf B stars (sdBs) is still unclear. About half of theknown sdBs are in close binary systems for which common envelope ejection isthe most likely formation channel. Little is known about this dynamic phase ofbinary evolution. Due to the tidal influence of the companion in close binarysystems, the rotation of the primary becomes synchronised to its orbitalmotion. In this case it is possible to constrain the mass of the companion, ifthe primary mass, its projected rotational velocity as well as its surfacegravity are known. For the first time we measured the projected rotationalvelocities of a large sdB binary sample from high resolution spectra. Weanalysed a sample of 51 sdB stars in close binaries, 40 of which have knownorbital parameters comprising half of all such systems known today.Synchronisation in sdB binaries is discussed both from the theoretical and theobservational point of view. The masses and the nature of the unseen companionscould be constrained in 31 cases. The companions to seven sdBs could be clearlyidentified as late M stars. One binary may have a brown dwarf companion. Theunseen companions of nine sdBs are white dwarfs with typical masses. The massof one white dwarf companion is very low. In eight cases the companion massexceeds 0.9 solar masses, four of which even exceed the Chandrasekhar limitindicating that they may be neutron stars. Even stellar mass black holes arepossible for the most massive companions. The distribution of the inclinationsof the systems with low mass companions appears to be consistent withexpectations, whereas a lack of high inclinations becomes obvious for themassive systems. We show that the formation of such systems can be explainedwith common envelope evolution and present an appropriate formation channelincluding two phases of unstable mass transfer and one supernova explosion.

Hot subdwarf stars in close-up view I. Rotational properties of subdwarf B stars in close binary systems and nature of their unseen companions