The International Astronomical Union (I.A.U.) appoints a committee that determines the names given to variable stars. The assignments are made in the order in which the variable stars were discovered in a constellation. If one of the stars already having a Greek letter name is found to be variable, the star will be referred to by the Greek name. Otherwise, the first variable found in a constellation would be given the letter R, the next S, and so on to the letter Z. The next star is named RR, then RS, and so on to RZ, SS to SZ, and so on to ZZ. Then the naming starts over at the beginning of the alphabet: AA, AB, and continuing on to QZ. This system (the letter J is always omitted) can accommodate 334 names. There are so many variables in some constellations in the Milky Way, however, that an additional nomenclature is necessary. After QZ, variables are named V335 (since 334 variables have already been named), V336, and so on. The letters representing stars are then combined with the possessive Latin form of the constellation name the same way that the Greek alphabet is used for complete identification of the variable star. Examples are SS Cygni (SS Cyg), AZ Ursae Majoris (AZ Uma), and V338 Cephei (V338 Cep).
Friedrich Argelander initiated this system of nomenclature. He started with a capitalized R for two reasons: the lowercase letters and the first part of the alphabet in capital letters had already been allocated for other designations, leaving capitals towards the lower end of the alphabet mostly unused. Argelander also believed that stellar variability was a rare phenomenon and that no more than 9 variables would be discovered in any constellation (which is certainly not the case!). Why the letter J is always omitted is a mystery lost in the dusty annals of astronomical history.