Hill Holliday chief executive officer Mike Sheehan has been named an unindicted co-conspirator in the case against former state treasurer ­Timothy J. Cahill and Cahill’s former campaign manager, Scott Campbell, a prosecutor said in court Monday.

Campbell and Cahill are ­accused of conspiring to use Cahill’s office for an unwarranted privilege: running ads for the Massachusetts State Lottery, allegedly to boost ­Cahill’s electoral hopes during his 2010 run for governor. Hill Holliday was the lottery’s longtime ad agency.