  The excess of far-ultraviolet (far-UV) radiation in elliptical galaxies has
remained one of their most enduring puzzles. In contrast, the origin of old
blue stars in the Milky Way, hot subdwarfs, is now reasonably well understood:
they are hot stars that have lost their hydrogen envelopes by various binary
interactions. Here, we review the main evolutionary channels that produce hot
subdwarfs in the Galaxy and present the results of binary population synthesis
simulations that reproduce the main properties of the Galactic hot-subdwarf
population. Applying the same model to elliptical galaxies, we show how this
model can explain the main observational properties of the far-UV excess,
including the far-UV spectrum, without the need to invoke ad hoc physical
processes. The model implies that the UV excess is not a sign of age, as has
been postulated previously, and predicts that it should not be strongly
dependent on the metallicity of the population.
