Sigma Orionis E is the prototype of this type of star's helium-rich class. High radial order g-modes produce the namesake phenomena of the slowly pulsating class of these stars.  Rapidly-rotating stars of this type that eject material in so-called decretion disks generate Balmer lines denoted by the designation "e," which is also used to label the more massive of the two classes of protostars named for Herbig.  In spiral galaxies, a tracer for spiral arms is a type of association consisting of recently-formed stars of this type and the next most massive spectral class.  Balmer line strength increases towards the cool 
Sigma Orionis E is the prototype of this type of star's helium-rich class. High radial order g-modes produce the namesake phenomena of the slowly pulsating class of these stars.  Rapidly-rotating stars of this type that eject material in so-called decretion disks generate Balmer lines denoted by the designation "e," which is also used to label the more massive of the two classes of protostars named for Herbig.  In spiral galaxies, a tracer for spiral arms is a type of association consisting of recently-formed stars of this type and the next most massive spectral class.  Balmer line strength increases towards the cool end of this spectral class, then peaks in the subsequent less massive class. FTP, the supergiant Rigel is a member of what spectral class that lies between the A and O classes?
Sigma Orionis E is the prototype of this type of star's helium-rich class.
B stars (accept helium-rich B or chemically peculiar B or slowly pulsating B or SPBs or Be or Herbig Be stars)