The class of variable stars are named after its prototype, FS Canis Majoris.
They are somewhat poorly understood, but are probably binary star systems in which mass exchange is ongoing or has recently happened.
These most likely consist of at least one B-type main-sequence star in a dust envelope.
Several have been detected in massive star clusters and this has ruled out some theories about their origin.
The preferred explanation is still for a binary origin, possibly as the result of stellar mergers.