tal right is bur- dened, a rational basis test is used to deter- mine the legitimacy of the classification. See Massachusetts Board of Retirement a Murgia, 427 U.S. 307, 311-12, 96 S.Ct. 2562, 2665-66, 49 L.Ed.2d 620 (1976) (per curiam). Here, the scope of the investiga- tion was limited to recently-r