Britney Spears just turned 40 last week, but she and fiancé Sam Asghari, 28, are keeping the birthday vibes going! While in Cabo San Lucas, Mexico, Brit and her soon-to-be-hubby were photographed on Monday morning as the they were settling in to a hot tub at their gorgeous resort. Britney rocked a sexy red string bikini, showing off her exceptional physique, pulling her beach blonde tresses into a high ponytail and sporting dark sunglasses.