  We report results of an all-sky search for periodic gravitational waves with
frequency between 50 and 510 Hz from isolated compact objects, i.e. neutron
stars. A new hierarchical multi-stage approach is taken, supported by the
computing power of the Einstein@Home project, allowing to probe more deeply
than ever before. 16 million sub-threshold candidates from the initial search
[LVC,arXiv:1606.09619] are followed up in three stages. None of those
candidates is consistent with an isolated gravitational wave emitter, and 90%
confidence level upper limits are placed on the amplitudes of continuous waves
from the target population. Between 170.5 and 171 Hz we set the most
constraining 90% confidence upper limit on the strain amplitude h0 at 4.3x10-25
, while at the high end of our frequency range we achieve an upper limit of
7.6x10-25. These are the most constraining all-sky upper limits to date and
constrain the ellipticity of rotating compact objects emitting at 300 Hz at a
distance D to less than 6x10-7 [d/100pc].
