  Giant low surface brightness (GLSB) galaxies are commonly thought to be
massive, dark matter dominated systems. However, this conclusion is based on
highly uncertain rotation curves. We present here a new study of two
prototypical GLSB galaxies: Malin 1 and NGC 7589. We re-analysed existing HI
observations and derived new rotation curves, which were used to investigate
the distributions of luminous and dark matter in these galaxies.
  In contrast to previous findings, the rotation curves of both galaxies show a
steep rise in the central parts, typical of high surface brightness (HSB)
systems. Mass decompositions with a dark matter halo show that baryons may
dominate the dynamics of the inner regions. Indeed, a "maximum disk" fit gives
stellar mass-to-light ratios in the range of values typically found for HSB
galaxies.
  These results, together with other recent studies, suggest that GLSB galaxies
are systems with a double structure: an inner HSB early-type spiral galaxy and
an outer extended LSB disk.
  We also tested the predictions of MOND: the rotation curve of NGC 7589 is
reproduced well, whereas Malin 1 represents a challenging test for the theory.
