A member of the group of pulsating variable stars that show some periodicity in their variations but are nevertheless unpredictable. The magnitude range is typically only one or two magnitudes, and the periodicity can range from a few days to several years. A number of subgroups have been distinguished. Types A and B are red giants, type C red supergiants and type D giants and supergiants of spectral types F, G and K. An example is Mu μ Cephei, known as the Garnet Star.