1509 = 3 x 503.
1509 is the number of disconnected 2-regular graphs on 39 vertices (A165652).
1509 is a number n such that 2n + 1, 4n + 1, and 8n + 1 are all primes (A124041).
1509 is a pinwheel number, satisfying the equation 2n2 + 6n + 1 for n = 26 (A059993).
1509 is a number n such that 1 + n + n3 + n5 + n7 + . . . + n33 + n35 is prime (A124187).
Source: On-Line Encyclopedia of Integer Sequences