  We present the discovery of an optical accretion disk wind in the X-ray
transient Swift J1858.6-0814. Our 90-spectrum data set, taken with the 10.4m
GTC telescope over 8 different epochs and across five months, reveals the
presence of conspicuous P-Cyg profiles in He I at 5876 Angs and Halpha. These
features are detected throughout the entire campaign, albeit their intensity
and main observational properties are observed to vary on time-scales as short
as five minutes. In particular, we observe significant variations in the wind
velocity, between a few hundreds and ~ 2400 km/s. In agreement with previous
reports, our observations are characterised by the presence of frequent flares,
although the relation between the continuum flux variability and the
presence/absence of wind features is not evident. The reported high activity of
the system at radio waves indicates that the optical wind of Swift J1858.6-0814
is contemporaneous with the radio-jet, as is the case for the handful of X-ray
binary transients that have shown so far optical P-Cyg profiles. Finally, we
compare our results with those of other sources showing optical accretion disk
winds, with emphasis on V404 Cyg and V4641 Sgr, since they also display strong
and variable optical wind features as well as similar flaring behaviour.
