  We present a study of the nebular phase spectra of a sample of Type
II-Plateau supernovae with identified progenitors or restrictive limits. The
evolution of line fluxes, shapes, and velocities are compared within the
sample, and interpreted by the use of a spectral synthesis code. The small
diversity within the dataset can be explained by strong mixing occurring during
the explosion, and by recognising that most lines have significant
contributions from primordial metals in the H envelope, which dominates the
total ejecta mass in these type of objects. In particular, when using the [O I]
6300, 6364 Angstrom doublet for estimating the core mass of the star, care has
to be taken to account for emission from primordial O in the envelope. Finally,
a correlation between the H-alpha line width and the mass of 56Ni is presented,
suggesting that higher energy explosions are associated with higher 56Ni
production.
