These stars vary with periods of 1 to 100 days and Henrietta Swan Leavitt discovered that they display a Period-Luminosity relationship.
The Blazhko effect is a variation in one property of a subclass of these entities, which have periods usually under one day. That same subclass is located in the Horizontal Branch of the H-R diagram and is notable for having all representatives in a cluster show the same magnitude. Eta Aquilae, discovered in 1784, was the first one of these discovered, while their namesake was discovered by John Goodricke in the same year. Unlike (*) RR Lyrae, another population II version of these variable stars varies with a similar "shark fin" shaped light curve and is called W Virginis. These stars vary with periods of 1 to 100 days and Henrietta Swan Leavitt discovered that they display a Period-Luminosity relationship. FTP, name this class of intrinsic variable stars which was first observed in the Delta star of the constellation for which it was named. 
The Blazhko effect is a variation in one property of a subclass of these entities, which have periods usually under one day.
Cepheid variables [accept RR Lyrae before Eta Aquilae]