We investigate the properties of optically passive spirals and dusty red
galaxies in the A901/2 cluster complex at redshift ~0.17 using restframe
near-UV-optical SEDs, 24 micron IR data and HST morphologies from the STAGES
dataset. The cluster sample is based on COMBO-17 redshifts with an rms
precision of sigma_cz~2000 km/sec. We find that 'dusty red galaxies' and
'optically passive spirals' in A901/2 are largely the same phenomenon, and that
they form stars at a substantial rate, which is only 4x lower than that in blue
spirals at fixed mass. This star formation is more obscured than in blue
galaxies and its optical signatures are weak. They appear predominantly in the
stellar mass range of log M*/Msol=[10,11] where they constitute over half of
the star-forming galaxies in the cluster; they are thus a vital ingredient for
understanding the overall picture of star formation quenching in clusters. We
find that the mean specific SFR of star-forming galaxies in the cluster is
clearly lower than in the field, in contrast to the specific SFR properties of
blue galaxies alone, which appear similar in cluster and field. Such a rich red
spiral population is best explained if quenching is a slow process and
morphological transformation is delayed even more. At log M*/Msol<10, such
galaxies are rare, suggesting that their quenching is fast and accompanied by
morphological change. We note, that edge-on spirals play a minor role; despite
being dust-reddened they form only a small fraction of spirals independent of
environment.Comment: Accepted for publication in MNRA