  More than half of the sources identified by recent radio sky surveys have not
been detected by wide-field optical surveys. We present a study based on our
co-added image stacking technique, in which our aim is to detect the optical
emission from unresolved, isolated radio sources of the Very Large Array (VLA)
Faint Images of the Radio Sky at Twenty-cm (FIRST) survey that have no
identified optical counterparts in the Sloan Digital Sky Survey (SDSS) Stripe
82 co-added data set. From the FIRST catalogue, 2116 such radio point sources
were selected, and cut-out images, centred on the FIRST coordinates, were
generated from the Stripe 82 images. The already co-added cut-outs were stacked
once again to obtain images of high signal-to-noise ratio, in the hope that
optical emission from the radio sources would become detectable. Multiple
stacks were generated, based on the radio luminosity of the point sources. The
resulting stacked images show central peaks similar to point sources. The peaks
have very red colours with steep optical spectral energy distributions. We have
found that the optical spectral index alpha_nu falls in the range -2.9 <
alpha_nu < -2.2, depending only weakly on the radio flux. The total integration
times of the stacks are between 270 and 300 h, and the corresponding 5 sigma
detection limit is estimated to be about m_r = 26.6 mag. We argue that the
detected light is mainly from the central regions of dust-reddened Type 1
active galactic nuclei. Dust-reddened quasars might represent an early phase of
quasar evolution, and thus they can also give us an insight into the formation
of massive galaxies. The data used in the paper are available on-line at
http://www.vo.elte.hu/doublestacking.
