  We want to study the temporal and spectral behaviour of HU Aqr in the X-ray
domain during different accretion states. We obtained spectra and light curves
from four different XMM-Newton pointings covering intermediate and low states.
The X-ray observations were accompanied with high time resolution photometry
obtained with the Optima and ULTRACAM instruments. On two occasions in May 2002
and 2003 HU Aqr was found in an intermediate state with the accretion rate
reduced by a factor of 50 compared to earlier high state measurements. X-ray
spectra in the intermediate state can be described by a model containing a
blackbody component and hot thermal plasma. Contrary to the high state the
ratio between soft and hard X-ray flux is nearly balanced. In agreement with
previous measurements we observed a migration of the accretion spot and stream
towards the line connecting both stars. The brightness of HU Aqr was further
reduced by a factor of 80 during two low states in October 2003 and May 2005,
where it was detected at a luminosity of only L_X = 4.7 * 10^(28) erg/sec .
This luminosity would fit well with an active coronal emitter, but the
relatively high plasma temperatures of 3.5 and 2.0 keV are more compatible with
residual accretion. We updated the eclipse ephemeris of HU Aqr based on the
eclipse egress of the accretion spot measured in various wavelength bands. The
(O-C)-diagram of the observed accretion spot eclipse timings reveals complex
deviations from a linear trend, which can be explained by a constant or cyclic
period change or a combination thereof. The quadratic term implies a period
decrease at a rate of \dot{P}_orb = -7..-11 * 10^(-12) sec/sec. In case the
observed period change reflects a true angular momentum loss, this would be a
factor of 30 larger than given by gravitational radiation.
