  Stellar population studies of early-type galaxies (ETGs) based on their
optical stellar continuum suggest that these are quiescent systems. However,
emission lines and ultraviolet photometry reveal a diverse population. We use a
new version of the STARLIGHT spectral synthesis code and state-of-the-art
stellar population models to simultaneously fit SDSS spectra and GALEX
photometry for a sample of 3453 galaxies at $z < 0.1$ with $NUV-r > 5$ that are
classified as elliptical by Galaxy Zoo. We reproduce $FUV$ magnitudes for 80
per cent of UV upturn galaxies selected using criteria from the literature,
suggesting that additional stellar population ingredients such as binaries and
extreme horizontal branch stars may have a limited contribution to the UV
upturn. The addition of ultraviolet data leads to a broadening of the
distributions of mean stellar ages, metallicities and attenuation. Stellar
populations younger than $1\,$Gyr are required to reproduce the ultraviolet
emission in 17 per cent of our sample. These systems represent 43 per cent of
the sample at $5<NUV-r<5.5$ and span the same stellar mass range as other ETGs
in our sample. ETGs with young stellar components have larger $H\alpha$
equivalent widths ($W_{H\alpha}$) and larger dust attenuation. Emission line
ratios and $W_{H\alpha}$ indicate that the ionising source in these systems is
a mixture of young and old stellar populations. Their young stellar populations
are metal-poor, especially for high-mass galaxies, indicating recent star
formation associated with rejuvenation events triggered by external processes,
such as minor mergers.
