R Coronae Borealis
The stellar classification of this star is C-Hd, and it is classified as a R Coronae Borealis variable. It is a carbon star with a hydrogen-deficient spectra that also shows evidence of s-process elements, including overabundances of strontium and yttrium, but no barium. This combination of properties is exceptionally rare; only one other example has been found as of 2012. The elemental abundances are explained as the result of a single neutron exposure event, which is difficult to reconcile with a conjecture that this may be a post-AGB-type star. In 1999, U Aqr was proposed to be a Thorne-Zytkow object, instead of being a simple R Coronae Borealis variable.