  New photometric, spectroscopic, and eclipse timing observations of the
eclipsing binary star HP Aur allow for very accurate orbital determinations,
even in the presence of a third body and transient starspot activity. The
eclipsing binary masses are determined to an accuracy of +/-0.4% and the radii
to +/-0.6%. The masses are 0.9543 +/- 0.0041 and 0.8094 +/- 0.0036 solar
masses, and the radii are 1.0278 +/- 0.0042 and 0.7758 +/- 0.0034 solar radii,
respectively. The orbital period in the outer orbit is accurately known for the
first time - 4.332 +/- 0.011 years. A comparison with current theories of
stellar evolution show that the components absolute properties can be
well-matched by the current models at an age of about 7 billion years.
