  We give a bound on the primes dividing the denominators of invariants of
Picard curves of genus 3 with complex multiplication. Unlike earlier bounds in
genus 2 and 3, our bound is based not on bad reduction of curves, but on a very
explicit type of good reduction. This approach simultaneously yields a
simplification of the proof, and much sharper bounds. In fact, unlike all
previous bounds for genus 3, our bound is sharp enough for use in explicit
constructions of Picard curves.
