A star that exhibits an irregular light-curve. There are two types, with very different characteristics: eruptive variables near the main sequence; and evolved pulsating variables.

Irregular eruptive variables are divided into three broad groups. The poorly studied variables, designated I, are subdivided into IA for early spectral types (O–A) and IB for those with intermediate to late spectra (F–M). Stars associated with nebulosity (IN) may vary by several magnitudes with rapid changes (up to 1 magnitude in 1–10 days), and are divided by spectra into types INA and INB. Finally, there are IS stars with rapid variations (0.5–1 mag. in hours or days), subdivided into types ISA and ISB, although this group is poorly defined. Stars with specific emission features are classified as T Tauri stars (INT), or in rare cases when nebulosity is not present, IT. When absorption indicates the infall of material, the star is a YY Orionis star, type IN(YY).

Slow, irregular, pulsating giants or supergiants are designated type L. Many are poorly studied, and may subsequently be found to be semiregular variables or other types. All have late spectral types (K–M, C, and S). Giants are generally classified as LB; supergiants with amplitudes of around 1 mag. are LC.