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Eva Mendes and Rihanna partied for a cause when they attended the annual DKMS Gala at Cipriani Wall Street in New York City, which raised $2.5 million for the fight against leukemia. Mendes chose a floaty white chiffon frock with a black sash by Prabal Gurung, while Rihanna went for a lacy gown with a plunging neckline by Louis Vuitton. Before the event, Rihanna tweeted, “Pullin up to DKMS Gala! Lights, Cameras…..ACTION!!!!! #Werk.” The singer was honored with an award recognizing her for work in raising awareness about the disease. Meanwhile, in Washington D.C., Michelle Obama shared a laugh with comedian Seth Meyers at the annual White House Correspondents’ Dinner, and over in Barcelona, Reese Witherspoon and Robert Pattinson got cozy at the Spanish premiere of Water for Elephants. See more photos from last night’s hottest parties in the gallery.