Distribution of the half-light radius in parsecs (left axis) and pixels of (right axis) versus the luminosity (top) and colour (bottom) of objects with spectroscopy in HCG 90 and 22. Blue pentagons show objects with spectra in HCG 90 and red circles in HCG 22. Solid symbols mark the UCD candidates in both groups. Unresolved objects in this figure will have sizes close to r_h = 0. Pixel to parsec conversion in both panels corresponds to the WFPC2 pixel scale (). Thus, HCG 90 objects will have twice the pixel value on the y-axis, because of the higher resolution of the ACS/WFC (). For comparison purpose, ACS (F475W-F850LP) colours were transformed to the WFPC2 (F450W-F814W) using GALEV (Anders & Fritze-v. Alvensleben 2003; Kotulla et al. 2009) for the metallicity range of our objects.