A semi-regular variable is a late- or intermediate-type giant or supergiant pulsating variable that changes in brightness in a more or less predictable way. Periods range from 20 to more than 2,000 days, accompanied by light variations typically of one or two magnitudes. Four types are distinguished. SRa stars, also known as Z Aquarii stars, have periods longer than 35 days, amplitudes less than 2.5 magnitudes, and fairly regular variability, making them similar to Mira stars but with a smaller amplitude. SRb stars, of which Z Ursae Majoris is a good example, have periods greater than 20 days and amplitudes of less than 2.5 magnitudes. Unlike their SRa cousins, however, their periodicity is sometimes broken by lacunae of slow, irregular variations or constant brightness. SRc stars, of which Betelgeuse is a famous example, are supergiants characterized by low amplitudes and occasional standstills. Members of subgroups SRa to SRc all of spectral types M, C, or S, sometimes with emission lines. SRd stars, on the other hand, are a mixed bag of yellow giants and supergiants (spectral types F to K), much hotter than the other semiregulars, covering a wide magnitude range (0.1–4 magnitudes), and prone to occasional irregularities.