** Shares of online advertising company Magnite MGNI.O rise 14.1% to $24.08

** Rosenblatt raises MGNI's PT to $39 from $18

** MGNI's benefit from the upcoming AdTech antitrust remedy/penalty phase is likely to be very substantial, very possibly as early as early 2026 - Rosenblatt

** Alphabet's GOOGL.O Google will face a trial in September on antitrust enforcers' proposals to make it sell off part of its advertising technology business to address the company's dominance over tools used by online publishers to sell ads

** A fairer marketplace would mainly benefit MGNI by improving win rate - Rosenblatt

** See share ex-Google as a reasonable proxy, arguing for a potential 9% increase in market share for MGNI, or $450 million, vs 2024 adj. EBITDA of $197 million - brokerage

** MGNI has risen 32.5% YTD, as of last close

(Reporting by Juby Babu in Mexico City)

((Juby.Babu@thomsonreuters.com;))