semiregular variables(sem-ee-reg -yŭ-ler) (SR variables) A heterogeneous group of giant and supergiant pulsating variables showing brightness variations that do not usually exceed one or two magnitudes, that have a noticeable periodicity ranging from several days to several years, but are also disturbed at times by various irregularities. The light curves have diverse shapes. Semiregular regulars have been divided into four subgroups: SRa and SRb variables are red giants of late spectral type (M, C, and S) either with relatively stable periods (SRa) or ill-defined periods (SRb) and are difficult to differentiate from long-period s002195a variables; SRc variables are red supergiants of late spectral type, such as Betelgeuse, Antares, and Mu Cephei; SRd variables are highly luminous yellow (F, G, and K) supergiants and giants.
semiregular variables[¦sem·i′reg·yə·lər ′ver·ē·ə·bəlz]
Variable red giant stars whose variation in brightness is repeated, but whose period and light curve may vary considerably from one cycle to the next; they have absolute magnitude of about 0 or -1 and quasi-periods of from about 40 to 150 days.