Elliptically averaged observed surface brightness profiles from the SDSS photometry, converted to the UBVRI system (black solid lines) together with uncertainties (grey regions). Conversion to the UBVRI system is done according to Blanton & Roweis (2007). For comparison, some earlier measurements are also shown (points with error bars; for references, see Sect. 4). The earlier measurements in UBVR are from elliptically averaged data, while I and outermost parts of V measurements by Irwin et al. (2005) have been made along the minor axis only. Left panel: major axis; right panel: minor axis. For clarity, the BVRI profiles have been shifted by the indicated values.