  In his unpublished preprint "Definable Valuations" Koenigsmann shows that
every field that admits a t-henselian topology is either real closed or
separably closed or admits a definable valuation inducing the t-henselian
topology. To show this Koenigsmann investigates valuation rings induced by
certain (definable) subgroups of the field. The aim of this paper, based on the
authors PhD thesis, is to look at the methods used in the preprint in greater
detail and correct a mistake in the original paper based on a paper of Jahnke
and Koenigsmann.
