  We report on 139La and 63Ccu NMR/NQR measurements in the high-Tc
superconductor La1.90Sr0.10CuO4 with Tc=26.5 K. Spin fluctuations probed by
139La spin-lattice relaxation (T1), continuously slow down on cooling through
Tc. We argue that spin-freezing and superconductivity are bulk effects in this
sample. Thus, both phenomena have to coexist microscopically. The distribution
of 139La T1 values at low temperature reveals a wide spread of spin fluctuation
frequencies in CuO2 planes. A simple estimate shows that Cu nuclei at sites
where electronic fluctuations are the slowest are not observable because of too
short relaxation times (wipeout effect). This means that the Cu NQR wipeout,
observed in this sample, can be explained primarily by slow magnetic, rather
than charge, fluctuations. This result does not rule out the connection between
wipeout effect and charge stripe order [Hunt et al., Phys. Rev. Lett. 82, 4300
(1999)], but it indicates that the relationship between both phenomena is not
straightforward. We argue that the wipeout fraction cannot define a proper
order parameter for a stripe phase, and cannot be used alone as a criterion for
its existence.
