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More than half of the sources identified by recent radio sky surveys have notbeen detected by wide-field optical surveys. We present a study based on ourco-added image stacking technique, in which our aim is to detect the opticalemission from unresolved, isolated radio sources of the Very Large Array (VLA)Faint Images of the Radio Sky at Twenty-cm (FIRST) survey that have noidentified optical counterparts in the Sloan Digital Sky Survey (SDSS) Stripe82 co-added data set. From the FIRST catalogue, 2116 such radio point sourceswere selected, and cut-out images, centred on the FIRST coordinates, weregenerated from the Stripe 82 images. The already co-added cut-outs were stackedonce again to obtain images of high signal-to-noise ratio, in the hope thatoptical emission from the radio sources would become detectable. Multiplestacks were generated, based on the radio luminosity of the point sources. Theresulting stacked images show central peaks similar to point sources. The peakshave very red colours with steep optical spectral energy distributions. We havefound 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 integrationtimes of the stacks are between 270 and 300 h, and the corresponding 5 sigmadetection limit is estimated to be about m_r = 26.6 mag. We argue that thedetected light is mainly from the central regions of dust-reddened Type 1active galactic nuclei. Dust-reddened quasars might represent an early phase ofquasar evolution, and thus they can also give us an insight into the formationof massive galaxies. The data used in the paper are available on-line athttp://www.vo.elte.hu/doublestacking.

Revealing a strongly reddened, faint active galactic nucleus population by stacking deep co‐added images