  The cluster Praesepe (age 650 Myr) is an ideal laboratory to study stellar
evolution. Specifically, it allows us to trace the long-term decline of
rotation and activity on the main-sequence. Here we present rotation periods
measured for five stars in Praesepe with masses of 0.1-0.5Ms -- the first
rotation periods for members of this cluster. Photometric periodicities were
found from two extensive monitoring campaigns, and are confirmed by multiple
independent test procedures. We attribute these variations to magnetic spots
co-rotating with the objects, thus indicating the rotation period. The five
periods, ranging from 5 to 84h, show a clear positive correlation with object
mass, a trend which has been reported previously in younger clusters. When
comparing with data for F-K stars in the coeval Hyades, we find a dramatic drop
in the periods at spectral type K8-M2 (corresponding to 0.4-0.6Ms). A
comparison with periods of VLM stars in younger clusters provides a constraint
on the spin-down timescale: We find that the exponential rotational braking
timescale is clearly longer than 200 Myr, most likely 400-800 Myr. These
results are not affected by the small sample size in the rotation periods. Both
findings, the steep drop in the period-mass relation and the long spin-down
timescale, indicate a substantial change in the angular momentum loss mechanism
for very low mass objects, possibly the breakdown of the solar-type (Skumanich)
rotational braking. While the physical origin for this behaviour is unclear, we
argue that parts of it might be explained by the disappearance of the radiative
core and the resulting breakdown of an interface-type dynamo in the VLM regime.
Rotational studies in this mass range hold great potential to probe magnetic
properties and interior structure of main-sequence stars.
