  SAX J2103.5+4545 is the Be/X-ray binary with the shortest orbital period. It
shows extended bright and faint X-ray states that last for a few hundred days.
The main objective of this work is to investigate the relationship between the
X-ray and optical variability and to characterise the spectral and timing
properties of the bright and faint states. We have found a correlation between
the spectral and temporal parameters that fit the energy and power spectra.
Softer energy spectra correspond to softer power spectra. That is to say, when
the energy spectrum is soft the power at high frequencies is suppressed. We
also present the results of our monitoring of the Halpha line of the optical
counterpart since its discovery in 2003. There is a correlation between the
strength and shape of the Halpha line, originated in the circumstellar envelope
of the massive companion and the X-ray emission from the vicinity of the
neutron star. Halpha emission, indicative of an equatorial disc around the
B-type star, is detected whenever the source is bright in X-rays. When the disc
is absent, the X-ray emission decreases significantly. The long-term
variability of SAX J2103.5+4545 is characterised by fast episodes of disc loss
and subsequent reformation. The time scales for the loss and reformation of the
disc (about 2 years) are the fastest among Be/X-ray binaries.
