A few (roughly 10%) of the central stars of planetary nebulae are, despite their much lower (typically ~0.6 solar) masses, also observationally of the WR-type; i.e., they show emission line spectra with broad lines from helium, carbon and oxygen.
Kasneje se je pokazalo da črte izvirajo od helija, plina, ki sta ga leta 1868 neodvisno spektroskopsko odkrila Janssen in Lockyer.