On Pólya groups of some non-Galois number fields

We prove two conjectures proposed by Chabert and Halberstadt concerning Pólya groups of $S_4$-fields and $D_4$-fields. More generally, the latter will be proved for $D_n$-fields with $n\ge 4$ an even integer. Further, generalizing a result of Zantema, we also prove that the pre-Pólya group of a non-Galois field of a prime degree, e.g. an $A_5$-field, coincides with its Pólya group.