  We present the accurate measurement of the frequency of the $7S-7P$
laser-trapping transition for three francium isotopes. Our approach is based on
an interferometric comparison to deduce the unknown laser frequency from a
secondary laser frequency-standard. After careful investigation of systematics,
with samples of about 100 atoms the final accuracy reaches 8 MHz, an order of
magnitude better than the best previous measurement for $^{210}$Fr, and opens
the way to improved tests of the theoretical computation of Fr atomic
structure.
