SIMBAD references

The first optical spectra of Th4-4, obtained by the author in 1970, showed strong continuum and broad HI emission lines. In other words, at this time the object looked like a Be star. Some years later, [OIII] emission lines appeared in the spectrum indicating the existence of a gaseous envelope. Today the optical spectrum of Th4-4 is quite similar to that of a moderate excitation planetary nebula. The magnitude of the object has dropped from V=14.4mag to 16.6mag , and the effective temperature of the central star has increased from 22000K up to 40000K within the last 30 years.