  We review some techniques to study the nature and size of the optical
continuum sources in multiple QSOs. We focus on the source originating the
events with several months timescale (the rapid variability source) as well as
the source that is responsible for the non-variable background component (the
background source). The techniques are used to study both the rapid variability
source in Q0957+561 and the main (compact) background source in Q2237+0305.
