6-arm blue grand design of NGC 309
  The geometry and physics of the spiral structure of the giant Hubble type Sc
galaxy NGC 309 is studied. A schematic of two patterns with three arms in each
is suggested for the blue spiral. The red and blue patterns form together a
grand design with two-fold symmetry. A possible gas-dynamics explanation of the
phenomenon is suggested which shows how the two-arm red spiral may induce the
formation of the six-arm coherent blue spiral.
  Key words: galaxies: individual (NGC 309) -- galaxies: spiral
