Lindsay Lohan's father Michael has sparked rumours he is engaged after his girlfriend was spotted wearing a diamond ring.Erin Muller - who has been likened to the Mean Girls actress - was seen wearing the new piece of jewellery as the couple was photographed at the Bridgehampton Polo Tournament in New York on Saturday.A source tells America's OK! magazine, "Michael recently proposed to his girlfriend Erin Muller. But they're keeping it quiet for the moment."Michael confessed earlier this year that he was shopping for rings for Muller. He told reporters at the time: "We're talking about getting engaged soon."

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