  The discovery of an excess of light in the far-ultraviolet (UV) spectrum in
elliptical galaxies was a major surprise in 1969. While it is now clear that
this UV excess is caused by an old population of hot helium-burning stars
without large hydrogen-rich envelopes rather than young stars, their origin has
remained a mystery. Here we show that these stars most likely lost their
envelopes because of binary interactions, similar to the hot subdwarf
population in our own Galaxy. This has major implications for understanding the
evolution of the UV excess and of elliptical galaxies in general. In
particular, it implies that the UV excess is not a sign of age, as had been
postulated previously, and predicts that it should not be strongly dependent on
the metallicity of the population.
