If we look at stellar spectra with much detail, we will always be able to find some small differences, so that, strictly speaking, each star is peculiar. Star spectra are like fingerprints that are never exactly the same.
However there are stars that are much different from the average and are catalogued in special classes. As an example, some stars show metallic lines so strong to be classified as F, but show also Balmer lines so deep and Calcium H and K lines so weak that these stars can be as well classified as A type.
Other stars show chemical differences, emission spectra or strong magnetic fields. Some of these peculiar classes are described in some detail starting from the links of the following table.

Peculiar Type Short Description
Wolf-Rayet stars (clic) Very hot, with emission lines
White Dwarfs (clic) Among the more famous we find 40 Eri B and Sirius B
Type N (C2) (clic) Carbon stars (like 19 Piscium)
Type S (ZrO) (clic) Stars with ZrO bands that add to TiO (c Cyg)
Am Type (clic) A stars with strong metal lines
Bp (Hg-Mn) Type (clic) B8-9 stars with strong Hg - Mn lines
Ap Type (clic) A-F0 stars with Si, Lanthanoids, Sr, Cr lines
Young G and K stars (clic) with still strong resonance 6708 Li line
Beta Lyrae (clic) Double star with mass transfer
Symbiotic stars (clic) Nebula around an old giant and white dwarf couple

Some peculiar stars were already recognized by Harward astronomers (Annie Cannon and Antonia Maury) but the first catalogue of strange stars was collected by Willemina Fleming from 1899 to 1911. She collected in her catalogue many novae, Be stars, binaries like b Lyrae, O stars with emission lines, R or N (carbon stars) type stars, S type (with ZrO bands), variables like R CrB.

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