Premieres of two new series, ABC’s “Sirens” and CBS’s “How’d They Do That?,” left the competition wondering how they did it Wednesday, as each finished a solid second in its respective time slot.

“How’d They Do That?,” a reality hour from Lorimar TV, deciphered a 12.3 rating, 19 share in Nielsen at 8 p.m. to uncover CBS’s best results in that hour with regular series (barring a two-hour “In the Heat of the Night” last November) since “Davis Rules” and “Brooklyn Bridge” faced football bowl game competition on both rival webs back on Jan. 1, 1992.

The show may have answered the question of how to compete against time-period champ “Unsolved Mysteries” (14.3/22), as “How’d” placed just 2 shares behind NBC’s veteran reality show among adults 18-49 and tied ABC’s tiring comedies. “Beverly Hills, 90210″ was a repeat.

The ABC Prods. drama “Sirens,” meanwhile, introduced itself to the 10 p.m. beat with an arresting 11.7/21, ranking second behind “48 Hours” (12.5/22) in households and actually winning the hour among adults 18-49, nabbing a 21 share to a 19 for the CBS news magazine and a 13 for “Law and Order.”

That gave ABC its best regular-season series results there since before the 1990-91 term, as “Sirens” cashed in on a whopping lead-in from “Home Improvement” (22.6/34) and “Coach” (19.2/30).

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