The significant feature of a Cepheid variable is that there is a relationship between two intrinsic parameters, one of which can be easily measured, while knowledge of the other parameter is required. These parameters are:

Period of brightness variation and luminosity.

Explanation:

The Cepheid variables are used as distance indicators. This requires estimation of periods and (usually) intensity-mean magnitudes in order to establish a period-apparent luminosity relation. It is particularly important for the techniques employed to be as accurate and efficient as possible.