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The star also posted a close-up of her toned torso as she leaned back with one arm resting on her chair's arm.
Earlier in the day, Hamlin appeared in two photos on her boyfriend's Instagram Story, which showed them sharing a lounge chair as they soaked up the sun.
Cool girl: After the lovebirds, who have an 18-year age gap, sunbathed in his backyard ahead of his 38th birthday on Wednesday, the teenage model shared a few risqué photos of herself in just a $43 and matching $20 thong from Boux Avenue
Abs: Additionally, the star posted a close-up of her toned torso as she leaned back with one arm resting on her chair's arm