From: Rabbi Moshe Sofer
To: Unknown recipient
A woman bound in levirate obligation to an apostate yavam who cannot be compelled to perform yibum or chalitzah is now an agunah; the yavam's daughter has come forward testifying that her father died. The Chasam Sofer rules that in such a severe case of iggun, multiple independent grounds for leniency combine to justify accepting her testimony, and instructs a beis din of three to convene, accept even second-hand testimony, and permit the woman to remarry, pending his countersignature.
Source: Shu"t Chasam Sofer EH II §89