The fractions of reflection effect systems detected in different subclasses of hot subdwarfs

ABSTRACT Hot subdwarfs with close low-mass M-type or brown dwarf companions usually show the reflection effect and often also eclipses. Through inspecting the light variations, we estimate the fractions of reflection effect systems detected in different subclasses of hot subdwarfs and constrain their possible formation channels. We noticed that none of the helium-rich hot subdwarfs were found with the reflection effect. Most of them might form through the merger channel. About 13 per cent of the He-poor hot subdwarfs on the extreme horizontal branch (EHB) with $T_{\rm eff}$ $\sim$ 25 000–33 000 K are discovered with the reflection effect. Nevertheless, the cooler hot subdwarfs located on the top of the EHB, those grouped on the bottom of the EHB, and those located above the EHB with $T_{\rm eff}$ $\sim$ 45 000–75 000 K exhibit lower fractions of reflection effect systems of about 2 per cent. These three subclasses of hot subdwarfs might contain higher fractions of single stars. Hot subdwarfs below the EHB also show a low fraction of reflection effect systems. They might have a higher fraction of stars with close white dwarf companions. A similar fraction of reflection effect systems to those on the EHB with $T_{\rm eff}$ $\sim$ 25 000–33 000 K is found in hot subdwarfs located above the EHB with $T_{\rm eff}$ $\sim$ 35 000–45 000 K. We also discovered that hot subdwarfs close to the Galactic plane might have a higher fraction of stars with the reflection effect than those at higher latitudes.