Their pulsations are non-radial, that is, they vary in shape rather than volume; different parts of the star are expanding and contracting simultaneously.
Improvements in photometry had made finding smaller changes in magnitude easier, and they had found that a high percentage of hot stars were intrinsically variable.
[6] Ten had been discovered by 1993, though Waelkens was unsure if the prototype was actually a member and recommended referring to the group as slowly pulsating B (SPB) stars.
Six stars, namely Iota Herculis, 53 Piscium, Nu Eridani, Gamma Pegasi, HD 13745 (V354 Persei) and 53 Arietis had been found to exhibit both Beta Cephei and SPB variability.
[10] The following list contains selected slowly pulsating B-type stars that are of interest to amateur or professional astronomy.