The plaintiff in a putative securities class action against radio ratings company Arbitron Inc. must turn over the names of 11 confidential informants, a federal judge in Manhattan has ruled.

Southern District Judge Paul A. Engelmayer ruled on Nov. 14 in Plumbers and Pipefitters Local Union No. 630 Pension-Annuity Trust Fund v. Arbitron Inc., 1:08-cv-04063, that the names of the informants were not protected by the attorney work product doctrine.