Helium-weak star

Helium-weak stars are chemically peculiar stars which have a weak helium lines for their spectral type. Their helium lines place them in a later spectral type than their hydrogen lines.

Helium-weak star

Helium-weak stars are mid-to-late B-class stars with weaker than normal spectral lines of neutral helium, compared to normal stars with similar hydrogen line strengths. These are considered to be an extension of the Ap/Bp chemically-peculiar stars with slightly hotter temperatures. They often show similar increased abundances of heavy elements. The mechanism of atmospheric stratification of elements is thought to be responsible for both types of chemical peculiarity.
This is a non-extensive list of helium-weak stars.

Star Name	Constellation	Apparent Magnitude	Class	Notes
20 Eridani	Eridanus	5.23	B8/9 III	α² CVn variable
30 Capricorni	Capricornus	5.38	B5 II/III
θ Hydri	Hydrus	5.50	B8 III/IV	Suspected Binary;
22 Eridani	Eridanus	5.53	B9 IIIpSi	α² CVn variable
HD 74196	Vela	5.61	B7 Vn	Part of IC 2391
HD 28843	Eridanus	5.81	B9 III	SX Arietis variable
12 Canis Majoris	Canis Major	6.08	B7 II/III	SX Arietis variable
HD 34797	Lepus	6.54	B7 Vp	α² CVn variable; binary
HD 35456	Orion	6.95	B9 II/III	Binary

Helium-strong star

A related class of stars have anomalously strong helium lines in their spectra, and are known as helium-strong stars. These are the more massive stars with classes of B1 to B3, compared to the helium-weak stars with classes of B5 to B9. The following are examples:

Helium-variable star

V761 Centauri is well-known for both the variability of its brightness and of its helium spectral lines. Stars of this type are referred to as helium-variable. This is thought to occur when one hemisphere of the star's atmosphere is helium-weak and one is helium-strong.