  The Optical Monitor (OM) on-board XMM-Newton obtained optical/ultraviolet
data for the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST),
simultaneously with the X-ray detectors. With the XEST OM data, we aim to study
the optical and ultraviolet properties of TMC members, and to do correlative
studies between the X-ray and OM light curves. In particular, we aim to
determine whether accretion plays a significant role in the optical/ultraviolet
and X-ray emissions. The Neupert effect in stellar flares is also investigated.
Coordinates, average count rates and magnitudes were extracted from OM images,
together with light curves with low time resolution (a few kiloseconds). For a
few sources, OM FAST mode data were also available, and we extracted OM light
curves with high time resolution. The OM data were correlated with Two Micron
All Sky Survey (2MASS) data and with the XEST catalogue in the X-rays. The XEST
OM catalogue contains 2,148 entries of which 1,893 have 2MASS counterparts.
However, only 98 entries have X-ray counterparts, of which 51 of them are known
TMC members and 12 additional are TMC candidates. The OM data indicate that
accreting stars are statistically brighter in the U band than non-accreting
stars after correction for extinction, and have U-band excesses, most likely
due to accretion. The OM emission of accreting stars is variable, probably due
to accretion spots, but it does not correlate with the X-ray light curve,
suggesting that accretion does not contribute significantly to the X-ray
emission of most accreting stars. In some cases, flares were detected in both
X-ray and OM light curves and followed a Neupert effect pattern, in which the
optical/ultraviolet emission precedes the X-ray emission of a flare, whereas
the X-ray flux is proportional to the integral of the optical flux.
