What two observable properties of a cepheid variable are directly related to one another?

Answer: At the period that's between its peaks of brightness and its luminosity.

Explanation:

The two observable properties of a cepheid variable that are directly related to one another at the period between its peaks of brightness and its luminosity. The period-luminosity relation allows us to determine its luminosity from the period between its peaks of brightness. In astronomy, a period-luminosity relation is a relationship linking the luminosity of pulsating variable stars with their pulsation period. The popular relation, for Classical Cepheid variables, is also called Leavitt's law