The stellar populations of low surface brightness galaxies

Near-infrared (NIR) K' images of a sample of five low surface brightness di sc galaxies (LSBGs) have been combined with optical data, with the aim of c onstraining their star formation histories. Both red and blue LSBGs have be en imaged to enable comparison of their stellar populations. For both types of galaxy strong colour gradients are found, consistent with mean stellar age gradients. Very low stellar metallicities are ruled out on the basis of metallicity-sensitive optical-NIR colours. These five galaxies suggest tha t red and blue LSBGs have very different star formation histories and repre sent two independent routes to low B-band surface brightness. Blue LSBGs ar e well described by models with few, roughly constant star formation rates, whereas red LSBGs are better described by a 'faded disc' scenario.