Main-sequence relation for galaxies coded according to their morphological type and HI gas content: Late-type galaxies (≥Sa) are indicated by black symbols, and early-type galaxies are shown by red symbols. Filled dots are for HI gas-rich objects (HI-def ≤ 0.4), and HI-deficient objects (HI-def >  0.4) are shown by empty circles. Star formation rates were derived assuming stationary conditions. The Y-axis on the right side gives the corresponding Hα luminosities corrected for dust attenuation. The solid line is the best fit obtained for gas-rich star-forming systems. The horizontal dotted and dashed lines indicate the corresponding SFR derived using the number of ionising photons produced by an O3 class III and a B0 class III star as derived using the model atmospheres of Sternberg et al. (2003). The typical uncertainty on the data is shown in the lower left corner.