  When neutron stars (NS) accrete gas from low-mass binary companions,
explosive nuclear burning reactions in the NS envelope fuse hydrogen and helium
into heavier elements. The resulting thermonuclear (type-I) X-ray bursts
produce energy spectra that are fit well with black bodies, but a significant
number of burst observations show deviations from Planck spectra. Here we
present our analysis of RXTE/PCA observations of X-ray bursts from the NS
low-mass X-ray binary HETE J1900.1-2455. We have discovered that the
non-Planckian spectra are caused by photo-ionization edges. The
anti-correlation between the strength of the edges and the colour temperature
suggests that the edges are produced by the nuclear burning ashes that have
been transported upwards by convection and become exposed at the photosphere.
The atmosphere model fits show that occasionally the photosphere can consist
entirely of metals, and that the peculiar changes in black body temperature and
radius can be attributed to the emergence and disappearance of metals in the
photosphere. As the metals are detected already in the Eddington-limited phase,
it is possible that a radiatively driven wind ejects some of the burning ashes
into the interstellar space.
