  The universality and linearity of the Leavitt law are hypotheses commonly
adopted in studies of galaxy distances using Cepheid variables as standard
candles. In order to test these hypotheses, we obtain slopes of the Leavitt law
using linear regressions of fundamental-mode Cepheids observed by the Optical
Gravitational Lensing Experiment project in the Magellanic Clouds. We find that
the slopes in $VI$-bands and in the Wesenheit index behave exponentially,
indicating non-linearity. We also find that the slopes obtained using
long-period Cepheids can be considered as universal in the $VI$-bands, but not
in the Wesenheit index.
