  We present a photometric study of I-band variability in the young association
Cepheus OB3b. The study is sensitive to periodic variability on timescales of
less than a day, to more than 20 days. After rejection of contaminating objects
using V, I, R and narrowband H-alpha photometry, we find 475 objects with
measured rotation periods, which are very likely pre-main-sequence members of
the Cep OB3b star forming region.
  We revise the distance and age to Cep OB3b, putting it on the self-consistent
age and distance ladder of Mayne & Naylor (2008). This yields a distance
modulus of 8.8 +/- 0.2 mags, corresponding to a distance of 580 +/- 60 pc, and
an age of 4-5Myrs.
  The rotation period distribution confirms the general picture of rotational
evolution in young stars, exhibiting both the correlation between accretion
(determined in this case through narrowband H-alpha photometry) and rotation
expected from disc locking, and the dependence of rotation upon mass that is
seen in other star forming regions. However, this mass dependence is much
weaker in our data than found in other studies. Comparison to the similarly
aged NGC 2362 shows that the low-mass stars in Cep OB3b are rotating much more
slowly. This points to a possible link between star forming environment and
rotation properties. Such a link would call into question models of stellar
angular momentum evolution, which assume that the rotational period
distributions of young clusters and associations can be assembled into an
evolutionary sequence, thus ignoring environmental effects.
