normalised transmitted counts through the Swift/UVOT filters for Vega (green) and for the theoretical model of an O9.5V star reddened with E(B − V) = 1.20 mag (blue). They have been obtained multiplying the flux density by the effective area of each filter and converting fluxes to count rates. The effective wavelengths for each filter are marked with gray dashed lines. The largest differences are found for the UVOT-uw1 and UVOT-uw2 filters.