The cluster Praesepe ( age \sim 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 - 0.5 M _ { \odot } – 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 84 h , 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 - 0.6 M _ { \odot } ) .
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 in Praesepe .
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 ( VLM ) 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 .