When a see is vacant and until the designation of a diocesan administrator, the governance of a diocese devolves upon the auxiliary bishop or, if there are several, upon the one who is senior in promotion. If there is no auxiliary bishop, however, it devolves upon the college of consultors unless the Holy See has provided otherwise. The one who so assumes governance of the diocese is to convoke without delay the college competent to designate a diocesan administrator.
Source: Code of Canon Law (Vatican.va)