An improved system of variability classification is used in the fourth edition of the GCVS, based on recent developments in classification principles and taking into account the suggestions of a number of specialists. Variability types are grouped according to the major astrophysical reasons for variability.
All of these classes include objects of a dissimilar nature that belong to different types of light variability. On the other hand, an object may be variable because of almost all of the possible reasons or because of any combination of them. If a variable belongs to several types of variability, the types are joined in the data field by a "+" sign, e.g., E+UG, UV+BY.
Despite considerable success in understanding stellar variability pro- cesses, the classification adopted in the Catalogue is far from perfect. This is especially the case for explosive, symbiotic and novalike variables, X-ray sources, and peculiar objects.
The new variability types (ZZO, AM, R, BE, LBV, BLBOO, EP, SRS, LPB) have been added in the Name-Lists 67- 77 and in the GCVS vol.V.
If a variable star belongs to several types of light variability simultaneously, the types are joined in the type field by a "+" sign (e.g., E+UG, UV+BY).