Astrophysicist explains science behind once-in-a-lifetime nova outburst that will light up the sky this year

[Igel]

7. Apparently "T" means it was the third variable star discovered in the constellation

Today, variable stars are catalogued in a few slightly different ways according to their order of discovery. The first variable to be discovered in each constellation was assigned the letter R and the Latin genitive of the constellation name, such as R Andromedae. The second discovered variable is dubbed S, continuing in this fashion up to Z, beyond which two-letter names are introduced, such as RR Lyrae. The RR designation is followed by RS up to RZ, and then SS to SZ, and so forth up to ZZ. As more variable stars are discovered, the scheme returns to AA to AZ, BB to BZ, and up to QQ to QZ. Interestingly, the letter J is omitted in this scheme to avoid confusion with the letter I.

https://www.iau.org/public/themes/naming_stars/