[1] On the history of Merovingian institutions, Havet's conclusions were widely accepted (see La Formule N. rex Francor).
He aimed a further blow at Fustel's system by showing that the Frankish kings had never borne the Roman title of vir inluster, and that they could not therefore be considered as being in the first place Roman magistrates; and that in the royal diplomas the king issued his commands as rex Francorum and addressed his functionaries as viri inlustres.
His attention having been drawn to questions of authenticity by the forgeries of Denis Vrain-Lucas, Havet devoted himself to tracing the spurious documents that encumbered and perverted Merovingian and Carolingian history.
At the Bibliothèque nationale, where he obtained a post, he rendered great service by his wide knowledge of foreign languages, and read voraciously everything that related, however remotely, to his favourite studies.
Posthumously, his published and unpublished writings were collected and, with the exception of Les Cours royales des Îles normandes and Lettres de Gerbert, were published in two volumes called Questions mérovingiennes and Opuscules inédits (1896), containing important papers on diplomatics and on Carolingian and Merovingian history, as well as a large number of short monographs covering a variety of subjects.