  In the context of the Hubble Space Telescope UV Survey of Galactic Globular
Clusters (GCs), we derived high-precision, multi-band photometry to investigate
the multiple stellar populations in the massive and metal-poor GC M 15. By
creating for red-giant branch (RGB) stars of the cluster a 'chromosome map',
which is a pseudo two-colour diagram made with appropriate combination of
F275W, F336W, F438W, and F814W magnitudes, we revealed colour spreads around
two of the three already known stellar populations. These spreads cannot be
produced by photometric errors alone and could hide the existence of (two)
additional populations. This discovery increases the complexity of the
multiple-population phenomenon in M 15. Our analysis shows that M 15 exhibits a
faint sub-giant branch (SGB), which is also detected in colour-magnitude
diagrams (CMDs) made with optical magnitudes only. This poorly-populated SGB
includes about 5% of the total number of SGB stars and evolves into a red RGB
in the m_F336W vs. m_F336W-m_F814W CMD, suggesting that M 15 belongs to the
class of Type II GCs. We measured the relative number of stars in each
population at various radial distances from the cluster centre, showing that
all of these populations share the same radial distribution within statistic
uncertainties. These new findings are discussed in the context of the formation
and evolution scenarios of the multiple populations.
