Variable stars are those stars that change their apparent brightness (or sometimes their color) over time. They are unusual enough to have their names more well-coordinated within the astronomical community than "normal" stars. Unfortunately, for historical reasons, the naming scheme is more complicated than it needs to be. The first variable star to be discovered in a constellation, if it doesn’t already have a Greek-letter name or a Flamsteed number, is called R followed by the genitive name of the constellation; thus, the first variable star discovered in the constellation of Cetus that didn't already have a Bayer or Flamsteed designation was named R Ceti. The second variable star discovered in that constellation is called S, the third T, then U, then V, then W, then X, then Y, and then Z. This covers the first nine variable stars discovered in any given constellation.
The tenth variable star to be discovered in a constellation is named RR. (RR Lyrae is a famous example among astrophysicists.) The next is called RS, then RT, then RU, then RV, then RW, then RX, then RY, then RZ. The next variable found after RZ is called SS, then the next ST, etc., through SZ; then TT through TZ, then UU through UZ, then VV through VZ, then WW through WZ, then XX through XZ, then YY, then YZ, and finally ZZ. This covers everything up through the 54th variable star discovered in that constellation.
Then, we go back to the front of the alphabet and start labelling variable stars as AA, AB, AC, . . ., AZ; then BB, BC, BD, . . ., BZ; then CC, CD, CE, . . ., CZ; then DD through DZ; then EE through EZ; and so on, until we get all the way down to QZ. This covers the first 334 variable stars in that constellation.
Note that the first letter of any of these two-letter names is never "later" in the alphabet than the second letter There can be an AB Canis Minoris, for instance, but there is not and never will be such a star as BA Canis Minoris. Nor will there ever be an HE Geminorum, or a GB Serpentis, or an XY Reticuli.
If still more names are needed, the next variable stars are numbered more rationally, starting with V335, then V336, then V337, etc..