  The NO cycle takes place in the deepest layer of a H-burning core or shell,
when the temperature exceeds T {\simeq} 30 {\cdot} 106 K. The O depletion
observed in some globular cluster giant stars, always associated with a Na
enhancement, may be due to either a deep mixing during the RGB (red giant
branch) phase of the star or to the pollution of the primordial gas by an early
population of massive AGB (asymptotic giant branch) stars, whose chemical
composition was modified by the hot bottom burning. In both cases, the NO cycle
is responsible for the O depletion. The activation of this cycle depends on the
rate of the 15N(p,{\gamma})16O reaction. A precise evaluation of this reaction
rate at temperatures as low as experienced in H-burning zones in stellar
interiors is mandatory to understand the observed O abundances. We present a
new measurement of the 15N(p,{\gamma})16O reaction performed at LUNA covering
for the first time the center of mass energy range 70-370 keV, which
corresponds to stellar temperatures between 65 {\cdot} 106 K and 780 {\cdot}106
K. This range includes the 15N(p,{\gamma})16O Gamow-peak energy of explosive
H-burning taking place in the external layer of a nova and the one of the hot
bottom burning (HBB) nucleosynthesis occurring in massive AGB stars. With the
present data, we are also able to confirm the result of the previous R-matrix
extrapolation. In particular, in the temperature range of astrophysical
interest, the new rate is about a factor of 2 smaller than reported in the
widely adopted compilation of reaction rates (NACRE or CF88) and the
uncertainty is now reduced down to the 10% level.
