A helium star is an O star or B star in which the absorption lines of helium are abnormally strong and those of hydrogen are absent or weak. Extreme helium stars (also called hydrogen-deficient stars) show no trace of hydrogen, while intermediate helium-rich stars have hydrogen lines that are visible but weaker than in normal stars. The loss or depletion of the star's hydrogen envelope, leaving essentially an exposed helium core, may have happened because of a powerful stellar wind, as in Wolf-Rayet stars, or because of mass transfer to a close binary companion. "Helium star" is also an obsolete name for a normal B star. A helium variable is a Bp star in which the strength of the helium lines varies periodically. At the extreme phases, the object appears as helium-rich, while at other phases helium lines may be very weak or absent.