A slow irregular variable (ascribed the GCVS types L, LB and LC) is a variable star that exhibit no or very poorly defined periodicity in their slowly changing light emissions. These stars have often been little-studied, and once more is learnt about them, they are reclassified into other categories such as semiregular variables.

Nomenclature

Irregular variable stars were first given acronyms based on the letter "I": Ia, Ib. and Ic.^[1] These were later refined so that the I codes were used "nebular" or "rapidly irregular" variable stars such as T Tauri and Orion variables. The remaining irregular stars, cool slowly varying giants and supergiants of type Ib or Ic were reassigned to Lb and Lc.^[2] When the General Catalogue of Variable Stars standardised its acronyms to be all uppercase, the codes LB and LC were used.^[3]

Type Lb

Slow irregular variables of late spectral types (K, M, C, S); as a rule, they are giants^[3]

The GCVS also claims to give this type to slow irregular red variables where the luminosity or spectral type is not known, although it also uses the type L for slow irregular red stars where the spectral type or luminosity is unclear. The K5 star CO Cygni is given as a representative example.^[3]

Type Lc

Irregular variable supergiants of late spectral types having amplitudes of about 1 mag in V^[3]

The M2 supergiant TZ Cassiopeiae is given as a representative example.^[3]

List

Other irregular variables

There are a number of other types of variable stars lacking clearly detectable periods, and which are sometimes referred to as irregular variables:^[4]

• γ Cassiopeiae variables, eruptive shell stars
• Orion variables, pre-main-sequence stars, including T Tauri stars and YY Orionis stars
• Rapid irregular variables, possibly similar to Orion variables with shorter period
• Poorly-defined irregular variable stars, of unknown type

In addition, many types of eruptive or cataclysmic variable are highly unpredictable.^[4]