A model for the population of helium stars in the Galaxy I. Low-mass   stars
  By means of population synthesis we model the Galactic ensemble of helium stars. It is assumed that all helium stars are formed in binaries. Under this assumption, single helium stars are produced by the mergers of helium remnants of components of close binaries (mainly, by merging helium white dwarfs) and by disruption of binaries with helium components in supernovae explosions. The estimate of the total birthrate of helium stars in the Galaxy is 0.043 yr$^{-1}$, their total number is estimated as $4 \times 10^6$. The rate of binarity in the total sample is 76%. We construct a subsample of low-mass ($M_{\rm He} \lesssim 2 \ms$) helium stars limited by observational selection effects: stellar magnitude ($V_{\rm He} \leq 16$), ratio of stellar magnitudes of components in binaries ($V_{\rm He}\leq V_{\rm comp}$), lower limit of the semiamplitude of radial velocity that is necessary for discovery of binarity ($K_{min}=30$ km/s). The parameters of this ``observable'' sample are in satisfactory agreement with the parameters of the observed ensemble of sdB stars. In particular, in the selection-limited sample binarity rate is 58%. We analyze the relations between orbital periods and masses of helium stars and their companions in systems with different combinations of components. We expect that overwhelming majority ($\sim 90$%) of unobserved components in binary sdB stars are white dwarfs, predominantly, carbon-oxygen ones. 
L. Yungelson and A. Tutukov