What specific characteristic of Cepheid variables did Leavitt find correlated directly with their absolute luminosity?

Answer

The period of pulsation.

Leavitt established that the period of pulsation—the time taken to cycle from bright to dim and back—was directly related to the star's absolute luminosity, making it possible to determine true brightness if the period was known.

What specific characteristic of Cepheid variables did Leavitt find correlated directly with their absolute luminosity?
measurementHubblegalaxyDistanceCepheid