  Selection of extreme objects in the data from large-scale sky surveys is a
powerful tool for the detection of new classes of astrophysical objects or rare
stages of their evolution. The cross-matching of catalogues and analysis of the
color indices of their objects is a usual approach for this problem which has
already provided a lot of interesting results. However, the analysis of objects
that are found in only one of the surveys, and absent in all others, should
also attract close attention, as it may lead to the discovery of both
transients and objects with extreme color values. Here we report on the initial
study aimed at the detection of objects with a significant UV excess in their
spectra by cross-matching of the GALEX all-sky catalogue with several other
surveys in different wavelength ranges and analyzing the ones visible in GALEX
only, or having extreme UV to optical colors (ultraviolet luminous objects). We
describe the methodology for such investigation, explain the selection of
surveys for this study, and show the initial results based on the search in a
small fraction of the sky. We uncovered several prominent UV-only objects
lacking the counterparts in the catalogues of longer wavelengths, and discuss
their possible nature. We also detected a single source showing an extreme UV
to optical color and corresponding to UV flare on a cool sdM subdwarf star.
Finally, we discuss the possible populations of objects that may be revealed in
a future larger-scale analysis of this kind.
