The results of long-term spectral observations were used to search for changes in planetary nebulae and emission-line stars. Significant increase of excitation degree is found in two objects: M1-6 and M1-11.
@article{arxiv.astro-ph/0607167,
title = {Spectral variability of planetary nebulae and related objects},
author = {L. N. Kondratyeva},
journal= {arXiv preprint arXiv:astro-ph/0607167},
year = {2015}
}