Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Council in Trullo; Often Called The Quinisext Council.: Canon XCIII

Canon XCIII.

If the wife of a man who has gone away and does not appear, cohabit with another before she is assured of the death of the first, she is an adulteress.  The wives of soldiers who have married husbands who do not appear are in the same case; as are also they who on account of the wanderings of their husbands do not wait for their return.  But the circumstance here has some excuse, in that the suspicion of his death becomes very great.  But she who in ignorance has married a man who at the time was deserted by his wife, and then is dismissed because his first wife returns to him, has indeed committed fornication, but through ignorance; therefore she is not prevented from marrying, but it is better if she remain as she is.  If a soldier shall return after a long time, and find his wife on account of his long absence has been united to another man, if he so wishes, he may receive his own wife [back again], pardon being extended in consideration of their ignorance both to her and to the man who took her home in second marriage.

Notes.

Ancient Epitome of Canon XCIII.

A woman who when her husband does not turn up, before she is certain he is dead, takes another commits adultery.  But when the man returns he may receive her again, if he so elects.

Compare in the Corpus Juris Canonici, Gratian’s Decretum, Pars II., Causa xxxiv., Quæst. I. and II.  Epistle of St. Leo to Nicetas.  Also compare of St. Basil’s canon’s xxxj., xxxvj., and xlvj.