by Elizabeth O. Waagen
Most variable star names are relatively straightforward - SS Cyg, OY Car, V4330 Sgr, even VSX J142733.3+003415 - or at least unambiguous. However, there is one small group that is ambiguous. This group consists of those variables that have the Greek letter μ (mu) or ν (nu) or the letters MU or NU as part of their name.
If we could always use the Greek letters themselves, there would be no confusion - μ CEN versus MU CEN is very clear. However, that is often not possible, so the Greek letter must be spelled out. In English, μ becomes mu and ν becomes nu. Now we have MU CEN versus MU CEN – which is which? Same for NU PUP versus NU PUP – who is who?
Since most computer-search algorithms (at least, those used by the AAVSO) are case-independent, using lowercase mu or nu isn't a solution. The GCVS uses a period (.) after the Greek letter, as in "mu. CEP". VSX uses "* mu Cep". Both of these are awkward, and sometimes are not compatible with software interpretation, and are not intuitive to observers unfamiliar with the convention.
The General Catalogue of Variable Stars (GCVS)  is the official reference publication on variable star names, and it uses the Russian spelling of Greek letters. However, in Russian, μ and ν are spelled mu and nu, so that is not a solution for those letters.
Following the GCVS convention, the AAVSO has decided to use the Russian spelling of all Greek letters in the AAVSO International Database. After discussion with Nikolai Samus of the GCVS team, it was decided that for μ and ν the spellings "miu" and "niu" would be used.
The data in the AAVSO International Database for all stars with m-u or n-u in their names have been checked and allocated to the appropriate star: miu or MU, or niu or NU.
WHEN YOU REPORT YOUR OBSERVATIONS OF μ OR ν, PLEASE USE THE SPELLING miu OR niu, as in "miu Cen" for μ Cen and "niu Cen" for ν Cen. You may use upper- or lower case letters.
If there is a number as part of the name, as in "delta2 Gru", please put a space between the letters and the number, as in "del 2 Gru".
By the way, when you are searching the International Variable Star Index (VSX)  for a Greek-letter star, you may put in the Russian or English spelling, the abbreviation or the full spelling. For example, "teta Aps", "theta Aps", "tet Aps", and "the Aps" will all lead you to the same star!
Here is a table of Greek letters, their abbreviation as used in the AAVSO International Database, the Russian spelling/pronounced-spelling, and the English spelling.